I use a MIDI [Control_Out Modulation] that maps indivdual Str# Fret sources to a CC that controls the growl parameter in a virtual synth. Gain and offset are tweaked for each string as well.
This allows me control growl merely by varying hand position.
I also run a CC into the SP that allows direct control and can cover up the fret mapping.
Hope that makes sense,
Have fun . .