Al, you’re really pushing the boundary here
I’ve been using brass sounds layered with a piano for some time now - mainly because I wanted to have everything on one keyboard, with no wild jumping between keyboard zones, but rather simply blend in my brass for individual phrases.
I usually simply use a fader pedal to my second sound - don’t need notes to continue past pulling the pedal back. In some cases (longer reverb or delay tails), I use the fader pedal only to control the brass sound and leave the effects tails unaffected, but usually, I simply have a “_SoloVolume” fader, with everything related to the solo sound routed to that one - the brass sound and any effects on it as well. For short phrases (think of the brass phrases in the Blues Brothers version of “Sweet Home Chicago”), this is mostly good enough.
For your specific case (playing notes that stay up even after the pedal has been pulled back), I wouldn’t go for a modification of velocity (you’d need a simple ReaJS script for that; simply multiply velocity with CC value / 127). You run the risk of getting somewhat funny notes with velocities of 20 or 30 whilst pulling back or pushing up. May create some madness with Kontakt scripting…
There is a pretty simple solution in Cantabile’s routing features: use the “Condition” segment of the “Edit Route” dialog (click “omni”) for the route to your brass sound. In this segment you can define that events will only be routed when a certain CC condition is met - I’d go for “CC 11 is greater or equal to 64”. Now your notes will only be played when you have your expression pedal pushed up beyond halfway. Or you could use a simple switch pedal (sustenuto or soft) - you don’t need expression for a simple on/off.
But with an expression pedal, you could fade out your piano sound at the same time - I have never felt the need for it, because it often sounds a lot more dynamic with brass and piano joining forces, but in other songs, you might want to use an inverse binding from your expression pedal to the volume of your piano, so that it gets faded out when you play your horns.
Going further, you could also use Conditions to alternate between piano and brass - no layer anymore, you simply trigger horns OR piano depending on your expression pedal. Cantabile should take care of tracking the release of your respective notes, so you don’t get stuck or cut-off notes.
Give it a try and let us know!