I’ve been interested in this for a few years, but haven’t got into anything public yet. I was actually looking at echoes and unity solutions too to do this. The prototypes I made were just feeding max from the gps and compass data from an iPhone using gyrOSC.
Echoes seems like a very good solution, I’d love to hear if others are using it.
The unity path, you can find lots of Pokemon Go type projects and base things off them too.
Cheers