Hardly a month after its publication, the latest British-American plan for a peaceful resolution of the 12-year-old Rhodesia conflict has seen a dramatic reversal in its initial grim prospects. It now appears to have at least a fair chance of success.

To the surprise of many observers here, the plan has stirred up a whirlwind of diplomatic activity across this racially explosive region, drawing all the countries from giant Nigeria to tiny Malawi in its wake.

The involvement of Nigeria for the first time is just one more sign of the rippling effect that the Rhodesia issue is having throughout the continent. Nigerian leader Lt. Gen. Olusegun Obasanjo arrives in Washington today on a state visit and his talks with President Carter will involved Rhodesia among other things.

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[WORD ILLEGIBLE] messages and envoys are [WORD ILLEGIBLE] from one capital to another [WORD ILLEGIBLE] southern Africa, with the secret diplomacy unleashed by the British-American proposals apparently centered here in Lusaka and, even less expected, the Malawian capital of Lilongue.

"There is going to be a lot of talking in the next few weeks," commented one Zambian official. He was unwilling to comment further.

After the furtive all-day visit of Rhodesia's white prime minister, Ian Smith, to this capital two weeks ago - despite the virtual state of war that exists between the two neighboring states - there is a feeling here that "anything is possible now," as an astounded Western diplomat put it.

Indeed, if 12 years of off-again, on-again negotiations over the breakaway British colony have proven anything, it is that appearances are highly deceiving, noes are never final, militant words should not be taken at face value and hope springs eternal.

After all, it was only nine months ago that Smith and the African nationalist leaders were competing in curses over abortive peace plan that bore the name of former Secretary of State Henry Kissinger.

The latest one calls upon Smith to hand over power peacefully to Britain, the former colonial power, and provides for a peaceful transition under British conglomerate Lonrho, which has extensive investments ththe end of 1978 through elections based on one man, one vote.

The latest cover of Africa Magazine pictures 14 African leaders and Western envoys now entangled in the settlement effort, with a suitcase bearing the names of nine capitals in the center. The headline, "Zimbabwe, Tedious Trek," seems to sum up - using the Rhodesia's Africa name - the unending search for a peace there.

Local Rhodesia watchers regard the secretive comings and going of a Swiss - registered executive Gulf Stream jet belonging to the private British peace entrepreneur, Tiny Rowland, as just the tip of the hidden diplomacy around the British-American settlement plan.

Rowland is chairman of the British conglomerat Lonrho, which has extensive investments throughout southern Africa and thus a direct interest in the making of peace in Rhodesia. He regards himself as a "revolutionary capitalist." It was a Lonrho plane that brought Smith here on his secret mission and Rowland probably arranged the meeting.

The Swiss registered jet was again sighted here this week but its passengers remain unknown, adding to the sense of mystery about what really is going on.

There has already been a lot of highly unusual diplomatic shuttling. In the past two weeks or so, South African Foreign Minister R.F. "Pik" Botha has visited Malawi for talks with President Hastings Banda and so, too, have two moderate black nationalist leaders of Rhodesia, Bishop Abel Muzorewa and the Rev. Ndabaningi Sithole.

Banda, a maverick black African leader who has quietly kept his ties to South Africa, apparently has offered to act as a go-between in what may well be an attempt to rally the moderate Rhodesian black leaders behind the leadership of Joshua Nkomo, co-leader of the Patriotic Front. It groups the two guerrilla forces fighting to overthrow Smith's government.

Nkomo appears to be emerging as the favorite candidate of both white and black leaders in the region, plus probably also of both the East and West, to become the first black president of Zimbabwe. He is acceptable to Smith and has strong diplomatic backing here and in many other African capitals.

Whether his chief rival, Robert Mugabe, who leads the more radical wing of the Front, will blow to his leadership remains one of the big unknowns. Officials of the Mugabe and Nkomo factions were meeting here this week to discuss a fusion of their political organizations, the better to deal with the British-American plan and head off a civil war later. But Mugabe failed to show up, leaving the outcome in doubt.

These meetings, like the Smith-Kaunda and Botha-Banda ones, bear testimony to the reverberations that the British-American proposal has caused in the region so far and seem to suggest that it being taken seriously after all.

At first, none of the interested parties had anything good to say about the plan. It met with reservations and conditions from the "frontline" African nations engaged in the issue, a rough "interrogation" from South Africa, an apparent outright rejection from the Patriotic Front and a scathing attack from Smith.

British Foreign Secretary David Owen and U.S. Ambassador Andrew Young left southern African Sept. 2, after a grueling ordeal of presenting the proposals to all the parties, barely able to mask their own discouragement. "Mission impossible" was how Owen at one point described it.

Since then, however, the frontline leaders have approved the plan as a basis for further negotiations. Nigeria has come out in support of it, the Patiriotic Front has called for more talks about it, and Smith has invited special representatives of Britain and the United Nations to visit Salisbury to discuss it.

In addition, the U.N. Security Council has agreed to the first step toward the involvement of the world body in trying to carry out the plan. On Tuesday, U.N. Secretary General Kurt Waldheim named the highly respected retired Lt. Gen. Prem Chand of India, who led peacekeeping forces in Cyprus and the Congo, to deal with implementation.

While these steps do not add up yet to a total acceptance of the plan by any party, they do indicate that the frontline states anyway regard it as the best hope yet for a way out of the increasingly bloody Rhodesian conflict.

"We could not reject them," said a Zambian official of the proposals, "because there would have been nothing better to replace them with."

Besides, as President Kaunda quietly boasted in one of his recent speeches, six of the seven main proposals were African-inspired in the first place.

Perhaps the best backhanded compliment to the British-American initiative has come almost unnoticed from Moscow. Last year, the Soviets virulently attacked the Kissinger plan as long as it was alive and kicking. This time, they have maintained their silence, apparently fearful of antagonizing their African allies.