Looking at the Pipewire source, it is making use of the same xdg-desktop-portal API as the (dropped) PulseAudio patches were:
https://github.com/PipeWire/pipewire/blob/master/src/modules/module-flatpak.c
Again, there is some flatpak specific code to check whether the client is likely to be sandboxed by Flatpak prior to the AccessDevice method call (this time checking for /.flatpak-info in the process’s mount namespace), so we would need changes there to support snaps.