my model of Open thread predicts that if I'm wrong, I'll be downvoted a few times, and may or may not get good feedback.

I think downvotes would depend on how you present your idea. If you present your idea as if you're already convinced you're right, and you're not, I think that would lead to downvotes. But if you preface your idea with "hey, here's something I thought of, dunno if it works, would appreciate feedback," I think that would be fine. What people respond negatively to, I think, is not wrongness so much as arrogant wrongness. (Or at least that appears to be what I respond negatively to.)

I doubt existing visitors to Less Wrong would be significantly more likely to generate crackpot ideas because of the existence of a thread

My model of the median LessWronger is closer to a crackpot than yours, maybe. Not that I think this is uniformly a bad thing; I have a vague suspicion that the brains of crackpots and the brains of curious, successful thinkers are probably pretty similar (e.g. because of stuff like this post). But it's easy to read the Sequences and think "man, I totally understand decision theory and also quantum mechanics now, I'm going to go off and have a bunch of ideas about them" and to be honest I don't want to encourage this.