Update (sorry to dominate the conversation).
Chris, thanks again for your timely reminder about bandolier pouches.
It got me thinking about big game fishing and the belts they wear. I'm not a fisherman, but looking at the images of their gear, I cobbled together a variation which in static test situations (as opposed to panning racing machinery) seems to be giving me a good advantage over straight hand-holding (at 420mm FF including 1.4x on Sig 120-300 S). I'm using a water bottle belt which has a pocket that the base of my Manfrotto monopod slots into, and a Chinese-made ball head, loosely tightened (if that's not an oxymoron). I think I will visit a fishing tackle shop to look at cushioned rod bases which might help the comfort factor.
Anyway, it's certainly worth taking to the Ferrari Festival next weekend for the acid test! What have I got to lose?
But I'm not giving up on my quest for a BushHawk yet.