Measuring voltage between two points...wirelessly?
There's no such thing as an "on-board stable potential" with reference to the outside world. All voltages are relative - a voltage is a potential difference - so in order to measure the voltage between two points, you either need to compare them directly, or you need a common reference potential to compare each to.

In a small area you might use neutral or an earth ground as your common potential, but these can vary over larger areas.

The obvious question is, if you don't have a potential you can use as a reference for both measurements, then how will the resulting measurement be meaningful or useful at all?