Ahh slight trail-of-thought fail in my original post 
The outputs from that circuit are supposed to feed into the CV input of one of the various voltage-controlled amplifier circuits I have coaxed out of Google - I haven't decided which one to build yet but the simplest one seems like a good place to start!
Perhaps something like this: Voltage Controlled Amplifier - Amplifier Circuit Design might be a good starting point to experiment with?
The one thing I can't figure out just by looking at the envelope generator schematic is whether or not the attack level will ALWAYS eventually go up to the maximum, or does it's max level depend on the peak voltage of the trigger pulse? Might be one of those "build it and see" things... if the output level is proportional to the input (e.g. for a very basic sort of velocity sensing on a trigger pad), then I'm in business. If not, then I'm stuffed and need something else.