I know that things are far from decided regarding height speakers, but if anyone would care to speculate ....
How important is matching speakers for the height channels to the other speakers?
Are height speakers to be direct or indirect radiating? I suspect direct, since there will be no nearby walls. On the other hand, I doubt that you would want them to be easily localizable.
In-wall speakers would be ideal, but most manufacturers would obviously not offer those, so if timbre matching is at all important, I guess they would be surface mounted, which would not be especially obtrusive if you have a CRT projector already mounted in the general area, but less than ideal if you have a digital projector mounted in the back.
Craig