Been studying a bit more and the SKREEM contains an RF transceiver and a microprocessor (with some memory I would think. Problem is we can't just remove it because it also controls the RKE and the power door locks.
And it is not a discrete signal, it is data blocks:
"Connected to the SKREEM is a Sentry Key Antenna Ring which surrounds the ignition lock cylinder. When the ignition is switched on, the Sentry Key Antenna Ring is supplied with power. A data block is transmitted inductively via the Sentry Key Antenna Ring to the SKREEM and then on to the Powertrain Control Module. If the antenna ring data block content is invalid or if vehicle battery power is too low to build up enough power for the antenna ring, the Powertrain Control Module will not receive the proper signal. This is displayed with the message “Start Error” in the Instrument Cluster"

OK then what we need to reverse engineer and reprogram the SKREEM to just send the data block which means we need to figure out the data block. No Big and may be easier than reprogramming the ECM.

Further: "The SKREEM retains in memory the ID numbers of any ignition key transponder that is programmed into it. For added system security each SKREEM is programmed with a unique secret key code. This code is stored in memory, sent over the CAN data bus to the PCM, and is encoded to the transponder of every ignition key that is programmed into the SKREEM." So what is needed is to reprogram the SKREEM not to interrogate the kay and just send the code.

Thoughts ?