yeah, both of mine basically 'rest' on the top rail, and then there's a bolt on the bottom that locks it into place so you can't lift off that rail.
I don't think it should matter that the rail is round or square or whatever, as long as it's angled a bit so you can lock hangers in place.
that image I found is from ikea hackers. I had a quick look at the whole thing, kind of interesting, but not sure I'd trust it the way they did it. I think I'd actually prefer the bottom hooks be flipped, so that the tv couldn't be lifted up off the top rail. I think that would make it much more secure
Displays: Samsung PN64F8500/JVC X35
AVR: Pioneer VSX-1018AH, 5.1 audio
Sources: HTPC(Mediabrowser), PS3, XBOX360, Wii, Sony DVP-CX995V
Control: Harmony One