Ok, I may need to try to fine tune it but I may have a solution, found totally by mistake.
Yesterday I was setting up my fv500h to work as an expression pedal. When I went to calibrate it, it was moving from 83-98..., so as a volume pedal it was lowering the volume while in heel down position, but not all the way, and up the volume in heel up. I realized that the knob for volume in the pedal actually controls the range of the pedal's perceived movement. I changed the knob and calibrated again and it was going from 0 - 98.
SO TWO POSSIBLE SCENARIOS:
1. Adjust the level of movement in the pedal so it has only limited volume and assigned it as a output volume control. Then the heel down brings your volume down enough and heel up becomes a boost, which is just full regular volume.
2. Using a pedal with this same procedure, but limit its perceived range, which I think the fv500h does. I mean, heel down is volume at 70, heel up volume at a 100. Connect your output to it and from here to the console. It would be become a post Ax8 "boost"
Again, these are just ideas as I have not tried them but I noticed this behavior with this pedal yesterday and it could be a partial solution to the "problem"