A Treasury tax specialist yesterday acknowledged that the Reagan administration's contingent tax proposals have little chance of passage, but he rejected out of hand an alternative put forward by Rep. Dan Rostenkowski (D-Ill.), chairman of the House Ways and Means Committee.

In testimony to a House Budget Committee task force, John E. Chapoton, assistant secretary for tax policy, reiterated support for a 5 percent surcharge on income taxes and a $5-a-barrel excise tax on crude oil beginning in 1986, if projected budget deficits don't fall below stipulated levels before then.

But when Rep. Thomas J. Downey (D-N.Y.) cited bipartisan opposition to this contingent tax plan, Chapoton admitted that "we have recognized the point you make."

He argued that even if Congress refuses to go along, the administration plan may force the House and Senate to come up with alternative tax increases.

Chapoton rejected, however, the most publicized alternative, a proposal by Rostenkowski to kill a number of tax reductions enacted in 1981 that are not scheduled to go into effect until 1984 and 1985.

The most significant of these reductions is indexing of the individual income tax to offset inflation, beginning in 1985. The Rostenkowski proposal also would halt prospective cuts in estate and gift taxes and in the windfall profits tax on oil, among other items.

"Our view is negative," Chapoton told the task force. In addition to reiterating strong administration backing for indexing, Chapoton specifically cited the oil breaks, along with some breaks for small business, as provisions the administration will fight to preserve.

The only provision about which he indicated any willingness to negotiate was the expanded deduction for charitable contributions for people who do not itemize their tax returns. "We might look at that," he said.