From what I've read recently when you wire into PTO to get the 1200rpm's the torque converter locks up in the transmission torque converter thinking the trans will be working. That is on the 08's.
I don't think the trans torque converter locks up wired for the 900rpm option. I forget what they call that one.
The question is what will this do to the transmission idleing for long periods of time..I assume it puts wear on it normally not had at regular idle.