On the Futaba 9C, there is another option.
The 9C has slider switches on the left and right side of the box. On the left side (VRD-D switch) it is located so it can be controlled by the index finger without removing your thumb off the throttle stick.

In ACRO mode, I put the Chnl 6 VPP servo on the VRD-D slider switch. This way the throttle stick is unnafected, so you would not need to relearn it as in HELI mode. The VRD-D also gives full variable control over each throttle setting, i.e. it is not 'slaved' to the throttle.

Any opinions on how well it would work?

Wade