Full Bridge SMPS With High No Load Voltage?

It has to act like a current source; persumedly you can alter the feedback to do that.

You could put an extra winding/diode on the output choke and feed that to the output, you would need another diode in the main output to stop it feeding back.

Sounds good. The other option is sense the i_out and switch the regulation voltage accordingly. You might find you've got almost all the bits in place to do that already.

NT

It's not uncommon to use some parallel source of high voltage that has a comparatively high output impedance. When the heavy arc current starts to flow, the high voltage effectively collapses to the voltage provided by the low impedance supply. This is sometimes done with additional windings on the main transformer, sometimes done as a completely separate circuit. In either case, you can add circuitry to actually switch off the HV when the arc starts if you like. It may or may not be necessary to prevent the HV output from inhibiting LV circuitry through negative feedback. Many such circuits don't have negative feedback, but run at a conversion ratio determined by feedforward (line input) only. Paul Mathews

That might be an interesting exercise in stability.