[jester710]

I play third here for sure because as it is, you're basically playing stud high, and it'd be worth chasing queens down with a 3 flush, an overcard, and all that money in the pot in high-only.

A big difference between this and high-only, though, is that here you're the only one that knows it's a high-only hand. Your hand looks like a low draw, and then it looks like a pretty scary low draw on 4th. I would try to bet/raise 4th, then continue firing on fifth.

As far as whether the aggressor would tread more lightly with just queens, his Q was the highest upcard and so likely to be best for high on 3rd, plus you said he gets out of line at times with high-only hands. I see nothing that makes me think he's stronger than queens (and if he has buried aces or kings it doesn't affect your hand too terribly much).