As far as I know, Safari is doing something along these lines already (I believe WebRTC connections only work there after requesting camera or microphone permissions).

An explicit permission would be nicer, possibly ("this site is requesting to establish direct connections" or similar).

Sounds like a job for an adblocker

Is it? An adblocker blocks content.

Controlling invasive technologies like camera use or file browsing cannot be realistically achieved by an adblocker.