Finally it wasn't an xorg setting. I managed however to make a proof of concept.
Remove the txt extension from the attached file and copy it to the Kindle and run it on ssh:
while the mouse is connected. It should display the cursor.
There is a "pause" every 30 seconds, as the mouse reconnects (the power management profile needs probably to be changed).
Anyway, please test it and report back.
If everything seems okay, I'll make a PR to integrate in Bluetooth-HID-passthrough.