Sorry to jump in late. What we at Frappe are planning is not to make another offline POS but to enable sync between 2 ERPNext instances. This way you can run a full offline ERPNext locally (on a vanilla Ubuntu machine) and then it can sync when its online.
This way, we don’t maintain multiple code bases, just one “online” POS. We have a plan to make this work before October where we are delivering this solution to a retain chain. Maybe that should solve most problems and not have the headache of maintaining multiple 3rd party apps.
Any enhancements in the existing “online” POS are always welcome.