I don't remember at the time I built my 9mm that the Hahn top loader had the BHO feature, I didn't think it did. I also don't remember it being held in by the bolt catch, seems as if it was just inserted from the top into the magwell. Sounds as if they have changed that one a bit for the better, I'd definitely give it a try. The set screw on the bottom loaders is really just waiting to come loose and there is a bit of up down adjustment in them.
They have always offered this version as far as I know. They used to have three... top, bottom and dedicated. Now I believe they only offer the bottom and dedicated.