Just use the velocities to modify the attack time. Lot of editing this is the way it's ment to be done ...
Try the so-called patches which allow to create 4 variations of an intrument. Patch selection can be done via the buttons or via midi and realtime response is good. Steps: copy layer, modify layers, allocate the layers to patches. Lot of editing...
There IS a way to modify instr params on the fly using midi sysex, but it will be non-standard and slow (ie useless for realtime control).
Nothing was simple back in the 90s
