The first round of porting a real application (Poor Maps) is done and I have extended the API accordingly. There is still work to be done to finish the port, but I think - in API terms of QML interface - lots of it has been done and should be possible to use in practice. Few bugs have been found and fixed in meanwhile.

So, if you wish to get hands dirty and add Mapbox GL-based map to your app, feel free to try. I'll be happy to help and extend / modify the API according to the requirements.