Caps from Passat/Arteon are not the same. AFAIK, mirror inserts are also not the same, but it's possible to install them in MK7.
Dimmable mirrors will work but require wiring to the rear-view mirror in cabin. No coding.
What's next? Well, I've seen how the blind spot works in Audi Q7 4M and I want it!
In Q7 you have a lot of ambient lights and you can select illumination color. And the blind spot system is working together with ambient lights. When the BSM light turns on, it also changes the door ambient lights from whatever-color-it-was-set-to to bright red.
We know that Golf MK7 does not feature RGB light, and the ambient lights are white (standard), red (GTI), or blue (R).
I don't know if that's an EU-Spec car thing but in my case (standard MK7 with white ambient lights) I can reverse the polarity of ambient strip and it changes color to red. Here's a demo (from 1:33):
So what I want to do? Use something like ATTiny to detect when BSM light is used, power a relay (not sure if this is the way) that will disconnect original wiring from the ambient light and switch it to BSM wiring. So as soon as the BSM light is active, ambient light strip will switch from white to red and will flash together with the BSM light in the mirror. Demo (from 1:45):
So that's my next project