Since you asked to hear from me..
It does seem that what you are really after is not modifying the track length as in the number of steps it is playing, but the duration that a track is taking to play. That is very different from changing the number of steps you are actually playing.
But my assumption may be wrong, so do let me know.
I think that the current solution of determining the number of steps in a track works quite well for most users, and the LEN factor mapped to the LEN encoder is also one of my personal favorites, ever since it appeared there I subscribe to Ripe’s comment on that one.