I would be worried about the branch rotting, but also it could get moldy, I've had a branch do that before, get moldy from being too damp (didn't have an orchid on it, tho). Leafmite's setup would work, b/c the mount is inorganic, so it couldn't decompose, or mold.
@ Leafmite, is there a pic of this? I would love to see it, I've seen something similar before, I think with live spag at the bottom, in the water dish, it was pretty. I have a lava rock I'd gotten for this same thing, just not sure which orchid I wanna put on it yet...
Def moss, and def with a sundew plant on there, as well as an orchid... Can't find a sundew plant tho..