If Secretary of State Al Haig meant to imply what he seemed to be implying on ABC's "Issues and Answers" the other day, a promising breakthrough in the administration's approach to the never-ending Mideast crisis may be at hand.

What Haig was implying is that the administration has accepted as inevitable the necessity of dealing, however circuitously and covertly, with the Palestine Liberation Organization.

It will, as they say, be denied. As candidate and as president, Ronald Reagan has consistently castigated the PLO as a vicious outlaw group with which he will have no truck. Henry Kissinger's 1975 commitment to Israel to do nothing that would confer any sort of formal recognition of the PLO (until that organization orecognizes Israel's right to exist) is no less an article of faith with the Reagan crowd.

On the contrary, the Reagan administration has declared global war on international terrorism in general, with the PLO at the top of its bill of particulars. And yet, if there is any logic in the strategy Haig laid out for "tamping down" the escalating violence in (and from) Lebanon, it has to lead inexorably to the opening up of some sort of channel between the United States and the PLO.

Why? Because when Haig speaks of the American purpose as "hopefully to achieve a cease-fire," he can only mean a cease-fire between Israel and the PLO -- or as he put it, "the two protagonists in this situation." Perhaps even more significant was his statement, "We must recognize the anguish associated with this terrible problem -- and that this anguish hits both sides."

As for the U.S. role, the secretary took note of two meetings the American special envoy Philip Habib had with Israel's Prime Minister Menachem Begin last Sunday. And in the next breath, he spoke pointedly of "active" American efforts to promote a cease-fire centering on the United Nations, the "European partners" and friendly Arab governments.

The United Nations, while it has played a useful peace-keeping role more than once in the Middle East, is not likely to be able on its own to mediate a cease-fire between Israel and the PLO. But it is one place to head for if you are trying to strike up a connection with the PLO. (It was former U.N. ambassador Andrew Young's misfortune to get caught trying to do just that; the uproar from Israel forced him out of the Carter administration.)

The European allies are somebody else you turn to; central to the so-called European "initiative" in the Middle East has been an effort to build a bridge between the United States and the PLO. The potential for constructive intervention by the more moderate Arabs, as a go-between with the PloY, is self-evident.

In short, if what's afoot is a delicate, multi-faceted U.S. diplomatic effort to arrange some sort of "moderation," if not absolute cessation, of the violence between the PloY and Israel, that strikes me as a far sounder approach -- and a far more effective form of pressure on Israel -- than delaying the delivery of F16 fighter planes. The diplomatic equivalent of being sent to bed without supper is not going to deter Begin as long as the PLO continues to shell Israeli villages.

Still less are indiscriminate Israeli air attacks on Lebanese population centers likely to deter the PLO, whose indifference to non-combatant casualties is nearly total. When the chief of Israeli Army intelligence freely acknowledges that one purpose of the bombing of population centers is to give the Lebanese public "something to think about," he is admitting to a form of terrorism -- and a useless one, at that.

The notion that a terrorized Lebanese population can bring pressure on the Lebanese government to restrain the PLO -- or somehow expel the PLO forces from their Lebanese sanctuaries -- presupposes that their is a Lebanese government capable of maintaining law and order. Phil Habib and a group of Arab League foreign ministers are working on that problem to some good effect, in their efforts to wind down the Syrian missile crisis.

Now a cease-fire in the Israeli-PLO "war" has been added to the Habib mission. It's a tall order, as long as the underlying Palestinian issue remains unresolved.

But something constructive may have already come out of the recent bloodshed. If, in fact, the Reagan administration has come to the recognition that the PLO is, for all its repugnance, a principal "protagonist," it follows that the PLO's acquiescence will have to be obtained through some channel or other if there is to be a reliable cease-fire.

And this in turn means opening up a channel between the United States and the PLO that could have far-reaching implications for the Mideast peace process.