My first thoughts would be to simply use a commercial isolated power module, perhaps 12vdc to 5vdc or similar, driven from the regulated dc voltage available on the input side.
Getting enough current to reliably drive an opto direct from the 350v dc rail is going to be either lossy or complicated.
But how about a capacitive charge pump driven direct off the high voltage secondary ?
Lets see, 100pF charged to 350v contains 6.125 microjoules.
If you can discharge it every cycle down into a low voltage electrolytic, at say 20 Khz, that should be able to pump 125mW with excellent efficiency.
At 5V, that works out to 24.5mA.
That should easily be enough for any opto isolator.
It should work well over extremes of duty cycle.
I have never tried this myself, or seen it anywhere else, but I can see no reason why it should not work very well.