Hi there, redirected here from recent Diy_Efi posting... Maybe I can help? maybe not, but here goes...
One of the things I've seen done is to get all of the proper inputs and handshaked signals ONCE, (so it starts) then set a DTC into the BCM (such as a B2945 for an open/short in the passlock sensor) As long as this DTC failed AFTER a succesful start, then the BCM will continue to send the fuel enable signal regardless of passlock sensor input. AND since this DTC is stored in non-volatile RAM in the BCM, tehnically you can even disconnect the battery and the DTC will STILL be present - keeping the engine starting! Of course you will need to wire in the BCM with the minimum of wiring (B+, GND, passlock sensor the of course Class 2 data spliced to PCM) THEN using the orig ign key cylinder (even just the whole TrailBlazer column sitting on the seats) jumpered temporarily to the correct circuits, get it to start, disconnnect the passlock sensor creating the open (setting the DTC B2945) and from then on the BCM will always send the fuel enable signal.

This all assumes the original ECM/PCM AND BCM is available of course, otherwise you will need to perform a Theft Deterrant relearn using a TECH2 and TIS (not neccessarily from a dealership, but very likely as few shops have service programming capability)You will also require a valid VIN from one of the TrailBlazer/Envoys.
PLEASE NOTE- This TD RELEARN will likely NOT be required if the BCM and ECM/PCM were taken from the exact same vehicle.

There MAY be one other solution with respect to providing the Passlock sensor input itself.(but this would STILL require a BCM be wired in) If you try reasearching some of the remote start systems out there- as the Passlock sensor is really just a pair or precise resistors, those resistors (and values) are often part of an remote start installation as it allows the vehicle to start/run without the keys in the ignition. So those details are "out there" however my own personal experience in these aftermarket remote start systems is quite limited.

Good Luck and hope this helps your efforts
Regards
WopOnTour