This is not supported. Yes, you could fiddle with things using Line-In on a CONNECT, but there will be a very frustrating time skew.

Since you have all the required hardware on hand, you could try this. Maybe you will enjoy the time skew, but it would irritate me. You can send the 870’s line level surround output to CONNECT’s  Line-In, then play Line-In on the 100’s.

if I wired the Left and Right rear speaker outputs from the Yamaha to an Sonos amp (that I don’t yet have)- could that drive the rear  speakers?   

Sure, but you’d have the same time skew.

Ultimately, only you know what sounds “best” (to you).

I know a fellow who uses a SONOS soundbar, surrounds, and SUB. He has bass, treble, and SUB set at max. To me the result is all flash and boom, but he loves it. Fortunately, I don’t spend any amount of time at that location.

the Yamaha has a latency adjust-  any chance that would allow me to even out the time skew?  would I connect the rear sonos speakers to one sonos connect or one connect for each?

Note that the resulting system will have some awkward operational characteristics. While you may be able to deal with these instinctively, a visiting aunt or uncle will be frustrated.

As you tinker with 870 latencies, voice sync will suffer.

CONNECT’s are stereo. You’d only need one.

point taken...

TV stations and many video sources do a poor job of voice sync. I’ve seen a few situations where the SONOS latency is constructive. (Mostly it is not constructive)

Initially, I missed the “rear speaker outputs”. This will not work without some sort of level shifter. Use the 870’s line level outputs that are designed for this sort of adventure.