Hi all,
Spurred on by recent advice, and a general need to bite the bullet eventually, I decided to just go for it and try and twist/step down from an over-balanced handstand - rather than rolling or relying on a wall (or doorway). So off I nervously went to the gym yesterday, found an empty studio, and put a few thick yoga mats down ...just in case.
However ...despite a number of attempts, I could not do it. I either couldn't get myself to go over, saving with a bend of the elbows, or ended up rolling out every time. My reflex actions would not allow it. At least I was unharmed ...and got to practice my handstand rolls 
After a small amount of reflecting on this, I tried a new tactic. One that turned out to work very well for me. I kicked up into a handstand, back to wall, and slowly rotated from left to right whilst maintaining balance at the wall. It took a few sets, and felt very strange at first, but I was soon quite comfortable rotating so my hips faced [degrees]90[/degrees] to the left and right. I was toying with the idea of doing it in a corner, to aid with balance until I felt more comfortable with it, but there was not a suitable one available.
I went back to the floor to have another go, and it worked like a charm. I spent about 20 mins playing at them, twisting left and right, eventually with no mats whatsoever. Probably wasn't that graceful, but I certainly felt safe in doing it. I noticed another interesting thing as well. When I am trying to balance a free handstand with no wall, I always underbalance/kick up too weakly. However when I was actively intending to (slowly) overbalance I had some really good hold times. Some of my best, in fact.
Not really advice - just sharing in case it helps someone.
Cheers,
George.