from firebase documentation it should be possible to send messages from the device to the app server (if app server is using XMPP protocol).
At the B4X forum I didnt found anything about it.
Are these upstream messages supported by the B4A and B4I librarys?
If not, is there a workaround suggestion to do so?
It shouldn't be too difficult to send upstream messages with JavaObject. However you will need to have a server that implements this specific protocol. If you already run a server why not connect the clients directly to the server?