An ingenious plan cooked up at year's end by Mexico and Morgan Guaranty Trust Co. -- with the blessing of the Reagan administration -- may slice the Mexican debt to banks by as much as $10 billion and mark a turning point in the Third World debt crisis.

"It's a writedown, voluntary to be sure, but still a writedown," said Rep. Charles Schumer (D-N.Y.). "Therefore, {Treasury Secretary James A.} Baker has repudiated his own plan, without actually saying so." And Richard Feinberg, vice president of the Overseas Development Council, added: "Baker has just joined the debt relief club: he's legitimized the concept."

The Baker Plan, offered by the Treasury secretary in October 1985, called for substantial new lending by commercial banks and development banks such as the World Bank to Third World countries, provided the borrowers promised substantial economic reforms in their own countries. It rejected the idea of debt forgiveness as unnecessary, and even counterproductive.

The new idea is that Mexico would offer to swap up to $20 billion worth of its old (and not-so-highly regarded) loans for $10 billion worth of new bonds. The new principal (but not the interest) will be backed by a special issue of U.S. Treasury "zero-coupon" bonds bought and paid for by the Mexican government.

The banks that choose to participate, in effect, would be writing off up to half of the value of their old loans to Mexico -- which roughly is what is said to be their real worth -- in exchange for a higher-quality piece of paper. The new bond would be marketable, because the collateral -- the Treasury zero-coupon bonds -- would be in a safe in the New York Federal Reserve Bank.

Thus, the scheme embodies a form of outright debt relief, or forgiveness, and that's why Schumer, Feinberg and others see it as major innovation. The weakness of the Baker Plan was that it would pile new loans on poor nation borrowers, rather than reducing their annual debt burden by canceling some of it.

Former State Department official Myer Rashish -- who suggested the basic idea of the plan to the Treasury and State Departments four years ago with no success -- says the underlying principle is not forgiveness, but "defeasance," meaning wiping out or annuling a contract: in this case, swapping old loans for better paper, a collateralized bond.

No matter: if the Mexican-Morgan Guaranty plan were to work to its full potential, it not only would eliminate $10 billion of Mexico's $53 billion outstanding debt to the banks, but also would cut its annual debt-service burden by about $1 billion a year for the next 20 years.

But in a troubled era in which everyone is naturally anxious for snippets of good news, it is easy to overstate the potential for the new plan.

Debt expert William R. Cline of the Institute for International Economics said, "I think this is a step in the right direction. It is not forced forgiveness, it is not a mandatory relief scheme." But he worries that if the concept takes hold that existing debt is worth only 50 cents to 60 cents on the dollar, "this could degenerate into a quasimandatory scheme."

Cline views the current proposal, essentially, as providing an "exit bond," primarily for "smaller, nervous banks who feel safer in getting out of Mexico" and who do not choose to join in any new lending ventures to that country.

But he thinks that few major banks will want to accept a 50 percent discount on their original loans to Mexico, because that will require them to take too large a loss. Moreover, he argues, they will have to wait 20 years for a payoff on the new bonds.

Both Cline and Robin Broad of the Carnegie Endowment for International Peace observe that even if the scheme proves helpful to Mexico, it is not adaptable to other heavily indebted borrowers, who don't have the reserves necessary to finance the purchase of U.S. zero coupon bonds.

Nontheless, the initiation of this new gimmick by Mexico and the Morgan bank could be a psychologically significant departure if governments, especially the United States, take leadership in dealing with the debt crisis. Why, one might ask, did this plan pop up now, with the blessing of the Reagan administration?

The illuminating Chat Canlas/John Cavanagh "Debt Crists Network Newsletter" observed just prior to the announcement that at least 10 debtor nations have stopped paying interest on their debts, and Argentina is threating to become number 11.

With the value of Third World debt plunging on secondary markets, two major regional banks -- the Bank of Boston and Washington's Riggs National Bank -- actually wrote off some of their loans. They did not merely add to loan loss reserves, as Citicorp did earlier last year: the Bank of Boston and Riggs declared some of their loans to be worth not a penny.

In this environment, the Reagan administration appeared anxious for any reasonable voluntary scheme that might pour oil on troubled waters.

For Mexico, it's almost a riskless proposition: for $2 billion in cash, it picks up 20-year zero coupon bonds with a face value of $10 billion. If it finds banks willing to "take a hit," it can use those "zeroes" to achieve a $10 billion reduction in its foreign debt, at the same time enhancing financial market confidence in its government. From the U.S. Treasury point of view, if the plan works, it might soften the hostile tone toward the United States exhibited in the November Acapulco declaration by eight key Latin States, which asserted their unity and the need to be more independent of Washington.

So a little calm might go a long way: current rumors suggest that Argentina may be readying to place a moratorium on paying interest on its debt. Later in 1988, Venezuela will have a presidential election that may install a populist who is pushing the moratorium idea.

The new Mexican plan, while certainly no panacea for Mexico or the rest of Latin America, may be better than nothing. Credit Mexico, the Treasury and Morgan Guaranty for not sitting there transfixed. As Robin Broad says, it shows that some banks and some countries are willing to take significant steps to resolve the debt crisis. But what is really needed is a multilateral initiative on debt relief, and no one has yet taken the lead to generate one.