When a body is considering a proposal and an amendment to that proposal, there are three alternatives -- original proposal, amended proposal, and status quo. The only procedure I've ever seen applied is this: an up-or-down vote on the amendment (repeated if there are several amendments), followed by an up-or-down vote on the (possibly amended) proposal.
It occurs to me that this is equivalent to an election with three (or more) candidates. The above procedure, then, is an odd sort of runoff system in which one of the alternatives (status quo) is guaranteed a place in the runoff. When there is more than one amendment under consideration, it looks like a maximally unbalanced single-elimination tournament, in which the original proposal is competing from the first round and the status quo gets byes all the way to the final.
My question: has any deliberative body tried using a different voting system here instead? It would seem that any single-winner system -- condorcet, approval, whatever -- could be applied.