Okay, got it working, here's the fix.
I used the passthru component as described in this post;
forum.linuxcnc.org/24-hal-components/6487-pid-pin-problems
Works great. I'm stoked to be able to adjust parameters from within Axis / PYVCP.
Here's what it looks like in my postgui HAL file;
loadrt Passthru count=2
addf Passthru.0 servo-thread
addf Passthru.1 servo-thread
net out-sig <= pyvcp.PWM_Scale => Passthru.0.in
net io-sig <= Passthru.0.out => pwmgen.0.scale
net out1-sig <= pyvcp.PWM_Offset => Passthru.1.in
net io1-sig <= Passthru.1.out => pwmgen.0.offset