Mark, thanks for the feedback, and I've been studying and evaluating my options since my last post above. There is much to explore, including whether I am going to try that Toslink switch 4x1 you describe.
I was reluctant to do so because part of this project was to be as minimalistic and wireless as possible, but as you describe, getting that sweet 5.1 into the Playbar seems to make a significant difference. Or in my case, 5.0 since I only ordered the Sonos Sub today and am hoping to receive it this Friday.
I could write 1000 words on my experience this last week, but I'll spare everyone. I will say that I've been experimenting on what my optimum movie file format is in order to enjoy the DD5.1, and I'm pretty sure the AAC track, whether 2.0 or 5.1, always down mixes to 2.0; and that AC3 5.1 is the audio format that will work.
What I'm not sure about yet, is whether if AAC 2.0 is Audio Track 1, and AC3 5.1 is Audio Track 2, whether the AC3 DD5.1 will get through reliably.
I've started using "Subler" to unselect Audio Track 1 (AAC 2.0) and select AC3 5.1, and it appears to work. I'm also trying to figure out what the downside is to leaving my movie files in that condition. Lots of experimentation going on.
As a reminder, I've got Apple TV connected wirelessly to my MacPro iTunes Collection, and right now I have the optical out from the ATV going straight into the Playbar.
Thanks for any other thoughts you might have and the best way to manage my movie files.