Imagine that there was a proposal for an Article V amendment and a sitting U.S. Supreme Court testified to either support or oppose the proposal. Most people would probably think that inappropriate.
How about a state Supreme Court Justice? I note this because at least one state Supreme Court Justice testified before the Senate in the 1960s to support an amendment modifying the “one-person, one-vote” cases. No objection was raised during the testimony. I’m unsure what people would think now though.