jabm111 OMG this works!! I was able to lock a flex! I haven't tried riding the car yet, but I bet it works.
Caveat: if you have Communauto on a secondary, low-priv user account, you must activate GrapheneOS Geocoder as instructed but in the main user account, as the other user won't have that setting available. It took me a while to realise this.
Merci beaucoup!!
PS: No need to disable any GOS security setting, everything works out of the box once the geocoder is enabled.