Well, considering we don't have a periscope on the Ar234, it might be difficult for HTC to come up with a way to work the guns on the B29. I think that's the major reason why they haven't even tried it. I'm not too broken up, mind you.
But the thing with normal guns is that the gun is right in front of you. It shoots out to where you aim. Only, in the B29 that's not so. You have 3/4 remote gun turrets, and if you're, say, in the port waist position, somebody in charge toggles the guns to work with your station, and you aim them. But in AH that means there are convergence issues. The guns are many feet away in both directions (front/back) so how do you code it so that the bullets actually hit their targets? Answer: It's hard.
Also, if all bomber guns are set to convergence of 500 yards, in a b29 you'd never hit anything outside or inside that range. Even using one single gun (which B29s can't) there are still convergence issues.
So you see it's not an easy thing to put in.