hey i knew about that feature, but since the tempo got locked to the audio file's tempo i never used it.
But now i'm thinking of something :
Use a rack to remap all incoming notes to the same note, and use that to trigger a click ; now you can grab the midi groove file, make it play the click, resample it, warp it accurately (that's where the click comes in useful), and use that audio clip as the master.
That way you're not stuck to the original tempo and it's accurate, plus you can save the click rack + midi groove as a live clip, and reuse that.
(maybe that's what you meant though

)
Still miles away from the regroove mixer, but that looks pretty usable....