Marcus, has your talk from the 2017 UK/IE MoodleMoot on this topic gone up anywhere? It's not on the official conference playlist and I recall it being a pretty good summary of the "minimum" work necessary to get an existing activity module working with MoodleMobile. While the docs on the above link are useful in a general sense, the tutorial doesn't cover this specific case, which strikes me as the most likely case developers are going to encounter. The example covers developing a MoodleMobile specific plugin using an existing web service.