For information, I have merged the PR from @AlexTrovato which brings the custom mappings to the Zigbee2mqtt integration!
I just launched a DEV build on the Docker tag which will be ready within an hour:
gladysassistant/gladys:dev
(The build: https://github.com/GladysAssistant/Gladys/actions/runs/2078334755)
I am open to feedback on the Zigbee2mqtt integration, to verify that it indeed brings the expected behavior for devices that were not managed before, and that it does not break the rest of Zigbee2mqtt of course 
Thanks to those who will test it!