Best approach might be to use a number of mix windows to cover each area, and a separate one (or the background) to project overall.

If you use a pair of layers to playback in each area, then a 3rd to do the masking over the other two, using any of the border/crop visual effects.

The overall image could be done in a number of ways:
Create an accurate mask - and use it over a playback layer (tricky)
or have another setup saved in a preset that offsets the mixes such that one layer will appear over all of them...

Using a master mask is tricky to re-adjust afterwards if things move or need adjustment.

Treating each area separately might work for you - it uses more layers and mixes - but might be ultimately more flexible.

If these options aren't feasible - talk to Richard directly as there might be some alternative methods...

Do contact me to discuss some options or if you need more help

Simon