In my situation, there's no need for change on the fly during note/word learning, the purpose of my recordings. People get a recording of (1) the finished product, (2) their own part, (3) the finished product minus their part. Some also like to have the other individual part tracks to sing with during learning. Once notes & words are learned, the recordings have done their job.

Your replies tell me there's good reason for the historical practice of leaving the addition of interpretive variations in tempo (and other stuff too for that matter) until after the note-learning is done. Probably it's not worth the work to put interp. into learning tracks, given the small number of learners who really have trouble re-learning rhythms. It makes sense to spend the effort to develop these people's rhythmic flexibility.

That said, I've been thinking about it since yesterday, and with Audacity I could generate click tracks of varying speeds, then do a lot of measure-counting and cutting 'n' pasting to make a varied-tempo click track. But for now I think life's just too short for that...so much harmony, so little time...