The adventure continues...
I finally had the opportunity to drive the car with the fault present. Some of the information I provided previously was erroneous, so here's an accurate picture of what's going on.
The car is basically going into limp mode... Tach cycles between 1000 and 1500 RPM. The motor stays in limp mode regardless of whether the transmission is in gear or park. Torque can be placed on the transmission up to the 1500 rpm point, then the motor kicks back. Check engine light does not illuminate. Fault would seem to be related to heat... only goes away once the car cools overnight (or at least for a long time).
Dropped the car off at a dealership (unfortunately after closing). Next morning (today) no codes present. Gave them a full rundown of the conditions. They drive/test/drive/test etc., but fault won't occur. They were not receptive at all to L5 sensor suggestion... insist it would code and that all sorts of other nasty side effects would occur.
Does the above information still support the original diagnosis, or does this change the landscape?