You could basically do what you want. Changing the guitar model and the tuning would be done by changing presets on the HD500, and to do that via MIDI is simply a matter of sending PC messages. So you wouldn't be controlling the Variax directly with MIDI, but rather you'd have to save the presets on the HD500 with the tunings and guitar models you want, and when you call that preset up, the Variax will change (you have to set the presets to force the Variax to change when setting them up).

Changing parameters of effects in real-time via MIDI is a bit more limited. As long as you don't want to get too carried away, it can be done in a workaround way, but the HD500 doesn't directly give you access to effect parameter via MIDI. What it does do, however, is allow you control the expression pedal controllers with MIDI. So you can assign whatever parameters you want to control to the expression pedals EXP1 and EXP2, and then control those with a MIDI CC. The other thing that you can do with the Variax is turn the tone and volume controls off while using the modeled guitars and assign those to control parameters in the HD500 preset. So in essence, those knobs become like expression pedals.