I think your workaround works up to a point: you probably can only store the volume in a Rackspace/variation for the volume master which is assigned to the widget.
If you don’t want to use one plugin per volume parameter and group them like explained by @schamass, that you can use one sigle pedal widget and make it produce the CC 27 signal it “grabbed” by assigning it to the CC 27 parameter of the MIDI in block connected to your Amp Plugin:
