All good solutions. Converting to a current sink is the simplest, but the other options are fine as well.
An expensive but simple solution is to use a high voltage opamp. If the power rating isn't high enough you can buffer it with an emitter follower.
You could also look at power regulators instead of opamps. For instance, I think there is a high voltage version of the LM317. If that has a high enough voltage rating you could use it configured as a constant current source. This is only convenient if you want a fixed current. There may be other more modern regulators that would be suitable although high voltage ICs are not that common.