Just received a 40000mAh aceyoon laptop power bank.
(Amazon lists it as 50000mAh, but apparently it’s ‘only’ 40000 
)
This thing outputs up to 8A at 12V, so it’s got more than enough oomph to power the A4 and AR together.
I took the Rytm out for an hour or two this evening, and it didn’t make a dent in the charge.
I’m just a headphone splitter (and possibly a keystep) away from 2-player jams in the woods.
I’m in Canada, so I had trouble finding other chargers that could output enough current to power multiple machines, but I think this will be more than good enough. The same company also does a 20000mAh version, which is probably fine for many hours of fun.
edit - Moog werkstatt is 12V too, so it is free to join the A4 on voltage-controlled adventures!