I’m also (super slowly) trying to make one of these, and wanna do similar.
If it helps I made some power discoveries on the pins here. Plus you can push ethernet through the OTG at about 500MB I think.
I think I asked some questions on the MEGAPI github page about their on/off switch (which is what I was using to power this) or maybe I didn’t … either way, I had a really nice little box there which could make a great cheap TV box, if I could do safe shut-down and control that horrific fan.
Also, have you tried anyuthing with libmraa, @theophile?