Only a seasoned cryptographer might have noticed, but it was another of those telling vignettes from The Federal City. The Eyebrow Telegraph was churning at full speed in the Senate hearing room.

As members of the Senate Appropriations subcommittee on agriculture fired questions at witnesses, staff assistants aimed subtle eye rolls and eyebrow arches toward the back of the room.

There in the back, scrunched into a chair and trying for all the world to seem invisible, a lobbyist named Robert A. Rapoza alternately tugged at his mustache and answered with eyebrow arches of his own. A thin grin occasionally punctuated the wires he was putting back onto the Eyebrow Telegraph.

What was happening was that questions Rapoza had drafted beforehand were being read into the record, as though the senators themselves had thought them up. And by indirection, Rapoza was scolding the Agriculture Department witnesses who otherwise give him scant time of day.

All of this is routine on Capitol Hill. Interest groups place questions with friendly legislators, write stemwinder speeches for lawmakers or quickly prepare "dear colleague" appeal letters that legislators use to make points when an issue heats up. The trick lies in finding a willing accomplice.

Rapoza, as the National Rural Housing Coalition's man in Washington, does all of these things unabashedly. What makes him different from other high-roller lobbyists is that he has no money to feed into political kitties, and that his cause -- housing of the poor -- has been under almost constant assault by the Reagan administration since 1981.

The surprise, probably, is that while the administration has cut about $2 billion -- that is, about half -- from subsidized rural housing programs, it has failed to eliminate the programs from the USDA budget as it has proposed to do in each of the last two years. The programs mainly offer low interest loans and rent subsidies to individuals and nonprofit housing organizations.

That failure is explained in part by Rapoza's ability to goad a large but often somnolent rural grassroots network into appealing to their senators and representatives for help. In 1982, in 1985 and again this year, in league with the influential home-builders' lobby, the coalition has blunted assaults on loan and rent-subsidy housing programs.

Rapoza's organization, made up of 250 member groups around the country, runs on a shoestring, with an annual budget of less than $75,000. He is its lobbyist, its braintruster, its newsletter writer, its cook and bottle washer.

Time after time, advocates such as Sens. Thad Cochran (R-Miss.) and William S. Cohen (R-Maine) and Reps. Matthew F. McHugh (D-N.Y.), Stan Lundine (D-N.Y.) and Henry B. Gonzalez (D-Tex.), prompted by Rapoza, have challenged administration plans to cut housing programs.

"We try to be prepared to stay ahead of the administration. We know their position on these housing programs. We attempt to piece together a coalition when we need it," Rapoza said. "But we don't often meet with senators. Much of it is walking the halls, talking to staff people, keeping the pot stirred."

Last year it was Cohen, from a state where Farmers Home Administration (FmHA) housing programs are considered a large success, who negotiated the deal that saved the programs from budget extinction. This year, with help from the coalition, Cohen again led the charge, announcing plans to resist any further cuts in the programs.

Rapoza, 35, joined the National Rural Housing Coalition in 1982 after work with the Massachusetts state housing department, the old Rural Housing Alliance in Washington and the Rural Coalition, where he continues as a consultant.

The ongoing battle over public subsidies for rural housing is not without paradoxes. Despite having moved about 2 million rural families into better housing over the past 15 years, and despite a loan default mark of less than 1 percent, FmHA programs are approached by the Reagan administration as an activity better left to private enterprise.

"There are still 2-plus million rural families living in substandard housing, with inadequate plumbing and drinking water," Rapoza said in an interview. "The poorest and the most isolated have been left behind."

"This administration simply wants to get the government out of this business. But there is little history to support their contention that the private sector will take care of this problem . . . . The FmHA can't even get rural banks to take part in the guaranteed operating loan program for farmers," he said.

So as the explication goes on -- and the eyebrows are not arched at this point -- Rapoza is really talking, he says, about fairness.

"The administration view is that the government doesn't have a role to care for people who can't care for themselves," he said. " . . . If we subsidize someone through the tax code to have a house at the beach and a house in town, it seems fair that we subsidize others to have safe and sanitary housing."