Well, I said I wasn't completely sure and your test proves rather conclusively that it isn't so.
Since sub-classing won't work and neither do helpers your only option then seems to be patching
TBits to add whatever you need.
Or, of course, creating your own
TNicerBits class.
Or if all you want is a
Count() function that can be done easily (in a helper): e.g. using
TBits.FindFirstBit() and
TBits.FindNextBit(), or walking through
Bits[] and checking. Not, perhaps, the best way to do it, but it should work.