Federal prosecutors, concerned about unresolved legal questions over procedures for committing defendants to mental hospitals, said yesterday they want John W. Hinckley Jr. to appear in court in person Monday to formally waive any objections to his commitment to St. Elizabeths Hospital.

Hinckley said in a court filing Wednesday that he waived his right to the Monday hearing, at which time he could have argued that he was entitled to immediate release from St. Elizabeths, where he has been confined since a jury found he was legally insane when he shot President Reagan.

Hinckley's waiver, and the hospital's finding earlier this week that he is severely mentally ill and dangerous, appeared to clear the way for Judge Barrington D. Parker to commit Hinckley to the hospital indefinitely.

In a letter to Parker yesterday, however, U.S. Attorney Stanley S. Harris said that Hinckley's waiver, as it stands now, does not assure that he would not return to court in the future and raise legal challenges to his commitment by Parker.

As a result, Harris said, Hinckley should be required to appear in court to personally waive any "objections he may have to the validity of his commitment."

In particular, the government said it wants Hinckley to waive any possible argument that he has a right to a jury trial on the issue of his commitment.

In additional court papers, the prosecution suggested that Parker follow a series of technical procedures, which it said it was proposing "in the interest of caution," to provide a "more secure" legal basis for Hinckley's indefinite commitment to the hospital.

The prosecution suggested that Parker make a specific finding that the hospital's report shows by "clear and convincing evidence" that Hinckley has a "severe, chronic mental disorder" and that Hinckley is a danger to himself and others.

Parker has not taken any action on Hinckley's waiver or the government's proposals.

Once the terms of the commitment are resolved, Hinckley will still have the right to return to court every six months and try to prove that he is ready for release.