You do not need the ECU to control the system. There is a switch box diagram on one of the other posts. The PCU has the deflation relief valve that controls the line pressure at 8psi allowing the wheel valves to dump. You also do not want to turn on "supply" without turning on the "control" solenoid first or air will blow out the PCU vent. You could separate the front and rear axle air lines but I would not try individual tires as that would be very dangerous. Also, using a switch box will allow you to over inflate the tires to 125 psi supply. They will not stay high after you shut off the air, but if the air lines are left pressurized for long periods of time, the hub air seals will not last long.
I am the applications engineer on this system. There is also a safety feature in the software that will automatically bump up the tire pressures if the vehicle speed is too high for the setting. You will not have this feature and could damage the tires if they are low while driving on the highway. All this being said, the use of of switch controller bypasses about 6 critical safety functions and is not recommended.