So I replaced the feedback divider with a scaled up resistor network, with the top resistor in the megohm series. I also added a high-current PNP pulldown at the gate, driven from three sections of 4093 output; collector to ground, ~150 ohms across base and emitter, also 10k pulldown on the gate.
Efficiency is up to 85% at 150V and 20mA output, which is a lot better. I could probably improve things a little further by actually grinding the math to select the "correct" values for the gate pulldown and base-emitter resistor. But it's good enough for now for the one-off project.
What's really killing me is the 4 ohm on state resistance of the 600 volt MOSFET...I think a 300V part would work OK here and probably drop the on state resistance below an ohm. I just don't have one in stock... ; )