One of the control parameters for each loop channel has
a name which takes the form
chan_.loop.start (and chan_.loop.length)
One can do something quite like this with considerable
ease by setting the length value to something short and
then grabbing the start value and cranking on it. Both my
"solo" and "improv ensemble" control maps on the Motormix
do just this for such occasions. You should have a look at
the manual chapter about setting up control maps. I like
having both the start and length values accessible for a
wider range of effects.
[I just realized that some version of this does, in fact, appear
on my recent release on c74 - which is an all-radiaL set, by
the way. I think you'd have to be intimately acquainted with
the source material to recognize it, however..