I've dropped and zener regulated (if you want to call it that) 265VDC for phantom on several two channel rigs, with no problems in over 6 years so far. It's relatively cheap to do, but here's the framework and the catch:
Two tube units drawing less than 10mA
Power One 265VDC 100mA PSU
Two channels worth of voltage divider / dropping networks. Two resistors, zener, and cap per channel. (If I remember correctly; still consuming first coffee).
You have to use at least a 10W resistor in top position (in case of any lower component failure), and you eat about 50mA from the 265VDC for 2 channels.
So, its an enormous amount of heat you add to your box, and you have to pay close attention to the various failure states and what will happen to current and voltage when calculating parts wattages, cap voltages, and actual divider output with zener failure towards open state. I don't remember if zeners tend to fail open or shorted, but you should plan for either.
Those #s are from memory, I'm sure there's a faster math wiz who can walk through the calculations for us. It keeps working fine in the few boxes I've done, but it still gives me a queasy feeling at times.
