incus config device add "$CONTAINER" wayland proxy \
connect=unix:"$XDG_RUNTIME_DIR"/"$WAYLAND_DISPLAY" \
listen=unix:/run/user/1000/wayland-0 \
bind=container \
uid=1000 gid=1000 mode=0660 \
security.uid=1000 security.gid=1000
incus config device add "$CONTAINER" gpu gpu
No need to mess with ID mapping or anything like that, apparently.
Works much better than waypipe for GPU-accelerated applications.