The fingerprinting risks, to a layman, seem to be a red herring?

Have the user consent occur before the point of enumeration.

Or lock it behind the user already having installed the pwa and require confirmation (i.e. a browser site gets a flat denied message, a installed PWA gets a permission prompt).

Sort of depends on Firefox supporting installing PWAs though..

For webserial this feels like it would make sense... WebUSB does feel like an overreach and too much.