Head tube length is not very indicative of the stack height on MTBs due to headtube angles and axle-to-crown fork length variation driven by fork travel and especially wheel size. (Obviously, 29er fork > 650b fork > 26" fork.) These 3 things vary far more than road bikes, which you can tell a lot about based on HT and eTT length.
The biggest MTB I have seen in person was a 6'6" guy riding a 23" Trek Superfly 100. It was a massive bike, but it fit him. I'm only 6'0" for reference. Looking at Trek's site, I now see that bike has a 14.5cm head tube.
Anyway, I agree with Grimlock. I think you could be chasing a ghost. Have you been to a shop and ridden 21" - 23" bikes?