So I've fitted the dimming mirror in place of the dipping one, and it already does what I want in terms of stray reflections.
And, being an Audi one, it uses the existing screen mount, so no change there. I did test it before installation, and it seemed to work OK, so (having extended the power wires) I completed the installation, and just have to use an Add-A-Fuse to provide a switched (ignition) line for this and the camera I want in the same location.
Roll on when the builders are done, so I can reclaim my cabbie from my son's drive, and continue!