I'm not psychic.
Are you referring to another topic, or is it a common project, I'm completely ignorant of?
The original post doesn't mention microcontrollers or anything else, just relays. I thought it was some sort of multiplexer/switch box.
If there's a microcontroller in there, just put the diode in series with the regulator. Use 12V relays with zeners across the driver transistors. That way, if the power is reversed, the relays will just turn on. No harm done. The good thing about zeners is they absorb more energy, than a plain old diode, turning off the relays more quickly, which reduces arcing on the contacts.