The guy I refer to in Post 6 has a PVI modulator that I installed.

Brad, ut's not you. My first two or three communications with PVI were easy and friendly. After that, several days went by with neither call-back nor email response, as though they had dropped off the earth. The eventual responses had enough info to figure things out, but were not very good from a technical point of view.

I've realized another difficulty for me with the Metra approach: there's about 500 msec of latency with the M3B, so if I take the audio directly from the source, then feed the video out to the TV, lipsync will be very poor. Worse, my situation has a signal going to one TV, then to another, so there are multiple locations where latency might happen and even add up.

Yup, PVI looks like the solution here. That's amusing since the client is minimizing the number of DirecTV boxes he pays for by having one run throughout the house via a PVI modulator. It hadn't occurred to me that another modulator would offer the simplest solution here, but it sure looks like that's the answer!