I don't think you have to pressurize that port to test it. Just have a hole in your plate where the valve is. Pack the valve with grease, screw a filter on the head, then pressurize the oil side of the equation until you get air out the hole. I assume your plate can pressurize the oil side? they typically leak oil to coolant which is why I always thought you pressurize that side when leak checking the bundle.
I wouldn't think it would require pressurized oil to test, air would suffice, so long as you have some sort of grease sealing it.
Or am I oversimplifying this?