I'm enjoying my M80 living room stereo set-up, but I'd be fibbing if I didn't admit that I got really used to all-channel stereo in my study. (I like the pseudo 3D sound coming from the rear/sides.)

I happen to have a spare channel of amplification and a spare VP100 laying around. But I only have a 2-channel pre-amp (or what is serving as a pre-amp, a Yamaha WXC-50).

So what I'm toying with is splitting the left/right pre-amp channels, merging them together for a single channel, running the merged channel into a volume control*, then to the amp, to the VP100 placed directly behind me (centered) on top of an armoire. This will be very inexpensive to add, so it's been gnawing at me to give it a shot.

* The volume control is just for a one-time adjustment so the single rear speaker is toned way down.

My questions are: (1) will this damage anything, and (2) how bad will this sound? (If it won't damage anything I'm going to try it, but I love to hear opinions.)

Not sure what you mean by "splitting the left/right pre-amp channels, merging them together for a single channel"
If you're thinking of using a splitter cable (i.e. one that takes a mono source and splits it into a R/L pair) that does not work in the other direction.
Your best bet is to buy a stereo to mono converter box (or just use the R or L channel to try it out).

Thanks rrlev, it felt too easy to actually work.
I wanted so much to make use of the VP100, but oh well.
I actually have two spare channels of amplification, so I'll probably just get additional speakers and split each front channel for the rears.