There is no sending unit. This is obvious because Keydiver can burn you a new eprom that changes the operation of the gauge. The stock boost gauge is just a guess made by the ECU as to how much boost/vacuum you are pulling based on the load on the engine.
I am not quite sure how the signal is sent from the ECU to the dash, but I have read posts from Keydiver saying the signal is PWM based across a single wire. I guess you could check continuity across whichever wire that is, and then try swapping out ECUs, and lastly try swapping out the dash.