it (could) be feasible without major modifications if, for example, the FW could manage a simultaneous pressure of 2 steps (i.e.: the 3rd and the 12nd) recognizing them as "starting point" (the 3rd) and "sequence end" (the 12nd) when the button "Sequence Lenght" is active.
mainly, it's a matter of adding simply the recognition of the dual pressure, since one is yet recognized (the latter, as sequence end).
so, actually you press "Sequence lenght" button and ONE step to determne the sequence ending step.
with the modification, you press SEQUENCE LENGHT button, and press simultaneously 2 steps to determine the sequence start & ending point.