That’s a cool idea. I’m interested in an answer to this question. 
I have no clue about the Garage Door Controller. Since RTSP is now in the mainstream production firmware release channel, I would expect that existing features like the Garage Door Controller still work, but I don’t use that (or even own Cam v3), so my speculation probably isn’t very useful. If you’re feeling adventurous, then you could try the production firmware update, see what happens, and report back to the community. If that nukes your Garage Door Controller feature, then I imagine you could re-flash older firmware.