Okay, so after having a spelunk through the source code, I have the impression that this MOS / quality analysis functionality is limited solely to feedback off RTCP supports.
I was under the impression from the documentation that RTPEngine does some stream analysis of its own, although in fairness, the documentation does not explicitly make this claim or the contrary one way or another. :-)
But that would certainly explain my problem. It seems I fundamentally misunderstood what it is that RTPEngine actually provides.