Thanks for the input, @oSoMoN, pretty much what I assumed.
I’ve tried to come up with a patch for snapd itself to add the required interface, a draft-ish gist is available here: https://gist.github.com/beidl/81c32310e0be83f8478cd470fa9b35f8
A sample application snap is available here, only using the speech-dispatcher plug:
It’s not much and doesn’t seem to be enough for full support as spd-say within the example snap tries to connect to the socket at $XDG_RUNTIME_DIR/snap.$SNAP/speech-dispatcher/speechd.sock instead of $XDG_RUNTIME_DIR/speech-dispatcher/speechd.sock,
which would’ve been fine if the snap.$SNAP subdirectory existed with a symlink pointing to the actual speechd socket (which seems to be the default behaviour with wayland plug-consuming snaps).
Manually creating directory + speechd.sock symlink results in a connection failure when starting spd-say (probably due to AppArmor denial or missing directory in the snaps namespace).