HZPiano CC#007, so couldn't see how to affect timbre without affecting gain (volume)
Hi David: it's the opposite - with CC#7 you control volume (gain) without significantly affecting the note character (timbre). I only go as far as ~ 10 % in overall change in gain in a range of velocities, to keep the effect under control without too much "jumpiness".
I have not tested thoroughly, but I assume that CC#7 affects everything that is sounding at the moment in a given MIDI instrument (it is a controller for any volume knob). You can set CC#7 events per midi channel, so it will not affect other instruments in backing tracks, I hope. Moreover, since the (small) change in volume is applied simultaneous with a new sounding note, psychophysically I do not perceive a change in volume on the previous decaying notes, although it is probably happening and it may become counterproductive in some circumstances... Experience will tell. And AFAIK, CC#1 would to the same but since in some applications people may already use CC#1 while CC#7 is almost forgotten, I prefer CC#7...
I think that @johanibraaten mentioned having bought VelPro, which a very good and comprehensive program to manipulate velocity curves, and it is able to do exactly this. The difference between my tool and VelPro is 40 USD, customer support, and good quality vs "whatever-works-well-enough" š