Ok, I'll probably get flamed for asking, but here goes. I have a client with a 5.1 system installed by another company. The fronts are in-wall, rears are in-ceiling. The center is an average-sized cabinet-style speaker that currently sits behind a 55" LCD on its stand inside built-in cabinetry.

Obviously the center speaker doesn't perform well in this configuration. I don't have the height in the built-in cabinet to raise the TV, and there's no place for an in-wall center. I haven't found a center speaker or passive LCR bar short enough to not obstruct the TV.

So here's my question: could I use the center channel pre-out of the AVR and feed that into the analog input of a short (<3" tall) powered sound bar?

I realize there would likely be lip-synch & volume control issues to overcome, but could this theoretically work?

Relevant equipment is Sammy 55" LED, Yamaha Aventage AVR, & C4 processor for control.

sounds like a bad idea.

How tall is the TV screen off the cabinet? I have used this in similar situations without a problem:

[Link: kef.com]

It could work but you are causing yourself more issues.
Rule #1 in designing a surround sound system is the front 3 speakers have to be tonally matched. You just destroyed that - you're only 1 step up from the center channel behind the TV.

The quickest & cheapest way would be to remove that center channel and set the Yamaha for no center. That way it can create a phantom center channel in processing. That would be a noticeable improvement right there.

The other way is to do what golden said and get a full sound bar in there.
A powered sound bar that big (3" tall) is likely to sound like crap. And customer will not be happy with it, since it will likely be a downgrade in overall sound he's getting now.
A passive sound bar powered from that Yamaha, will do much better. The KEF linked to above would be a good start. You could also use a Triad ONWALL NANO LCR 3.0. A few other companies make skinny passive sound bars as well.

Assumption: this is not going to be perfect. Every response saying it won't be perfect is obvious, but worse, such a response seems to say you don't have a better idea, but you're willing to tell cshepard that his idea won't work. The guys at this site are better than that.

A short sound bar won't match, as said. The Yamaha has its EQ system, which will help.

Phantom center channel is a GREAT solution unless people visiting you are speaker counters (that would be most of us!). They'll sense the center channel in the center.

Trust me, even though it doesn't sound reasonable, systems can have the speakers were way off perfect but the sound was not only great but seemed to emanate from the TV! In the middle of an install in my garage, I had only one speaker, four feet to the right of the display and three feet up, and I realized to my shock while watching a movie that the sound seemed to emanate from the middle of the screen... until I thought about it.

No no and no. You will not be able to control the lip sync issues well enough to have it fly. find and use a non powered LCR sound bar and set the two in walls to front presence or just use the center portion of the LCR.
There are a million LCR bars out there in any given size and if you cant find exactly what you are looking for you can have one customized from a number of companies.

Thanks guys. You reassured me of what I already knew I needed to do. I'll get a passive soundbar & make the current in-walls front presence channels.

One last thought before we go. You did not mention if you have room on the sides. Could you put two small on wall speakers on each side of the tv? Perhaps even coming up with custom bracket to mount them on each side of the tv. Get creative, use phantom or run.