Well, I finally did it. First in history? Powered sideways flips (I give it a shot of negative pitch when it's upside down).
At the end, I flipped it all around and then "bailed out" with stabilize, and it worked fine. My wife couldn't keep the camera on it though.
After this, I tried to see what would happen if I pulled it full pitch up, as I know it hits a limit when vertical according to Leonard. So... this is not a good thing to do on a heli anyway, it did all kinds of nasty things and I thought I'd lose it. Flipped to stabilize and it saved itself. It may have accumulated a DCM error because it seemed to have some "leans" for a while and then it was OK.
This may seem trivial to anybody who flies helicopters, but it's taken a lot of hard work to get to this point! We now have a working acro mode for flybarless helis, which includes access to full negative pitch with a softer setting available in Stabilize mode. Basically there's an "idle up" running in the APM.
Thanks to Leonard for the great stabilized acro mode!