Best way to regulate high voltage
100VDC is in the domain of Switch Mode power supplies, which, for example, phone chargers are. You can get a ringing-core SMPS down to a cubic centimetre or two, especially if the system doesn't require high degrees of isolation between high and low voltage sides.

I would look into buying or building a small ringing-core SMPS, or using those cheap crappy fake Apple mini cubes from China for 5V and then going to 3.3V if you are lazy. Those things are dangerous and should not be used in the mains, but if you open them up and make sure no shorts can happen by unfolding the innards or at least glueing some stuff up, you have a tiny 5V supply from 80V to 300V DC.