Hallo Russell,

I would try the following idea:




Using mains transformer seems to be overkill compared to your X2 capacitor solution. But it offers full isolation, which highly simplifies the implementing of 'wired-or' feature. With X2 capacitor only, touching of DC inputs would be dangerous!

At inputs you see obligatory varistors for surge and burst protection. 275V types (for both!) seem to be suited. Fuses prevent trouble if varistors had been damaged. Transzorb provides fine filtering. It should only be turned-on during an overvoltage condition. It's task is to protect LM317.
LM317 is connected as current source.
Electrolytic capacitor should be selected in order to achieve minimum turn-on delay, while at the same time guaranteeing stable switching, not becoming badly influenced by ripple.
Capacitance of electrolytic capacitor can be choosen rather small, I think. Use of full wave rectifier helps to allow this.

At input of transformer a X2 capacitor is connected in series. It's task is to prevent any DC current from continuously flowing through primary winding, when erroneously connecting DC signal to AC input. 1M resistor is needed to prevent dangerous potential across AC terminals, after disconnecting AC signal.

Lower fuse at DC input should be rated in such a way, that an eroneously applied AC voltage will make it blow. The resulting current through series resistor, diode and transzorb of DC current loop can be quite high and must be limited to a sane value by the series resistor. This resistor must also be able to tolerate heavy spike currents during surges and bursts. So, this resistor should be very robust.
Please take care, that both varistors can withstand 220VAC! Otherwise lower one can explode when eroneously applying 220VAC to DC input!!!

Use fast acting fuses with sand-filling!

Kai