Bangkok, Thailand — For the past 18 months, a series of discreet signals exchanged between Washington and Laos, Indochina's smallest nation, offered prospects for clear progress in accounting for American servicemen missing there.
The process would probably have been accompanied by normalization of relations between the two countries. But then an American adventurer named James 'Bo' Gritz came along late last year, trying to recover the missing from Laos in a dramatic foray, and the signals became hazier.
Southeast Asia is not a major priority for the Reagan administration, but there are said to be three Asian issues the President feels strongly about - Amerasian children, any prisoners of war who may still be held in Indochina, and servicemen missing in action (MIA).
The administration has in fact made it clear that it will not consider normalizing relations with the countries of Indochina until they show positive signs of willingness to help account for MIAs.
There are in fact only two servicemen still legally classified as missing in action. The Pentagon, despite protests from many relatives of the missing, classified 2,452 other military men who are unaccounted for as either killed in action (KIA), body unrecovered, or presumed killed in action.
Officials handling the problem stress that these are legal categories: ''We don't say that the two MIAs are still alive, and we don't say that the KIAs are all dead,'' said one of them. They do not exclude the possibility that some Americans may be held prisoner for unfathomable reasons in remote parts of Indochina, but the major emphasis of their work is on accounting for the dead.
Teams from the US government's Joint Casualty Resolution Center (JCRC) in Hawaii have been meeting Vietnamese officials quarterly since last December to discuss missing servicemen. The talks are proceeding slowly, though: The US is strongly opposed to Vietnam's intervention in Kampuchea, and the Vietnamese are bitter at what they term US ''collusion'' with China against their country.
Progress with Laos, on the other hand, has been much more consistent. In 1981 , two Republican congressmen visited the Lao capital of Vientiane. Their visit was described as a private initiative, but it clearly had the blessing of the White House, and resulted in some $75,000 of aid being given to a Vientiane hospital. The money came from US government disaster relief funds, which are exempt from the congressional ban on aid to Laos.
In September last year, the Lao accepted a delegation from the National League of Families, the principal organization of relatives of missing servicemen. The delegation met government officials in Vientiane, traveled to two crash sites, and were accompanied on their trip to the sites by members of the small US diplomatic mission in Laos. All the gestures were unprecedented, and on the delegation's return to the US, a member of the group told a congressional committee that the ''ball is now in the US court.''
The tempo of signals built up. In October, the US deputy assistant secretary of state, Daniel O'Donohue, visited Vientiane and made a series of concrete proposals to the Lao. The proposals, not revealed at the time, included:
* US subsidies for transportation of aid material sent by voluntary agencies.
* US assistance in selling off fighter planes flown by Lao pilots to Thailand after the Pathet Lao takeover in 1975. The US has in fact already found a bidder for the planes, and will turn over the proceeds of the sale, estimated at $50, 000 to $100,000 to the Lao government. There is just one slight problem:No one is quite sure how many planes there are. Estimates range from two to seven.
Mr. O'Donohue also told his hosts that if there was a consistent pattern of progress on accounting for missing servicemen - apparently defined as access to crash sites by JCRC teams - the US would be prepared to consult Congress with a view to lifting the ban on aid to Laos and would consider upgrading diplomatic relations from change d'affaires to ambassadorial level.
US officials thought they saw an indirect response to these ideas when restrictions on US diplomats in Vientiane were eased somewhat at the end of last year. Then they received indications that Laos was prepared to alllow a JCRC team to visit crash sites.
And then came Bo Gritz's privately sponsored expedition which aroused resentment in Laos. Early this year, ''the Lao obviously felt we were playing both sides against the middle,' said a US official, ''though we now feel they know we weren't.'' The US has in fact gone to considerable lengths to discredit the Gritz foray - a special issue of the mercenary magazine Soldier of Fortune, comprehensively debunking Gritz, seems to have been written with official assistance.
The process has not broken down completely since then, though.
A JCRC team visited Laos for two days in February. They had hoped to visit one of the crash sites the National League of Families' delegation had seen. But though JCRC officials say they ''got on well'' with their Lao counterparts, they add that the Lao also mentioned ''most emphatically,'' the Gritz raid.
US officials are convinced they are working indirectly with senior Lao leaders who see the desirability of closer ties with the West. They are unsure how much maneuverability these leaders have vis-a-vis their own colleagues, or their Vietnamese allies.
Mr. Reagan, too, has maneuverability problems. On some MIA-related issues he can rely on broad congressional support, from Republican conservatives like Sen. Strom Thurmond (R) of South Carolina through the liberal Illinois Republican Sen. Charles Percy or New York Democratic Rep. Stephen Solarz. Other members of Congress remain viscerally opposed to anything they suspect resembles buying bodies from Indochina. This is why all the moves the US has made so far have originated in the executive branch.