National Symphony Orchestra President Leonard Silverstein, a tax attorney, is heading the Reagan administration's in-depth study of possible changes in the nation's tax laws to encourage private donations for the arts and humanities. As a member of the President's Committee on the Arts and Humanities, Silverstein belongs to a working group that includes Treasury Secretary Donald Regan, Smithsonian Secretary S. Dillon Ripley and Librarian of Congress Daniel Boorstin. So far the group has met three times, and Committee Chairman Andrew Heiskell, former Chairman of Time, Inc., has attended from time to time. The president's committee was created to stimulate private support for the arts while federal funding is slashed, and tax tinkering is the key. Silverstein is finding it a tricky business. For one thing, these are hard times and when you try to increase giving for the arts, "People may tend to say social welfare is more important." Since the administration doesn't want to give up tax revenue--at least not for this purpose--Silverstein is looking for tax tricks that will motivate giving, yet be "revenue-neutral." He realizes it may not be politically acceptable to balance tax breaks that would stimulate arts-giving by eliminating other tax breaks. Anything that is done runs the risk of being interpreted as goodies for upper-incomers who, like it or not, are the folks who go in for the arts and are willing to support them. "We want to increase giving on a revenue-neutral basis that will be fair," Silverstein said. "Some ingenuity is needed."