Yeah, chairs. When I first considered it I thought, "No way, that's crazy" for just those reasons. Then I futzed with trying to attach to the wooden rail with no lip or use my tabletop tripod or folding the magic arm around into some kind of mini stand to put on the "rail" or table there.
I wasn't happy though 'cause none of that was secure. If I clamped on the rail too hard the wood could be damaged and could easily give way and putting it on some mini-stand meant that it was easy for someone to nudge it or, even, pick it up and toss it over on a whim. No likey.
Attaching to chairs means that there's a lot more weight on the chair end and even with equipment failure someone's going to have to intentionally pick up the chair and toss it over to get my mics to fall. On top of that my bag or I will be sitting on it, so they'll have to fight me too. (I can think of lots of permutations on the idiot scenario but they all involve an idiot and intention. And, regardless of what I do, if Idiot has decided my gear's going over, it's going over.)
FWIW, this won't work well in other parts of the GAMH because people want to get there and stay in the other parts. Here, sight lines are bad *and* people don't want to hang out in the light near the toilets.