Interesting! I know years ago when I was discussing buns with recurrent stasis with my vet (when Nino used to get it a lot) he mentioned that a long-term Protexin product could be given to increase fibre intake daily....I wonder if this is the sort of thing?! Or the other link you put on the thread above for the toothless bun? Same manufacturer - I think this is the stuff he was talking about.
They do say gastrointestinally rabbits and horses are the 'most' similar. No doubt colonisation of the caecal bacteria varies a little though....
maybe they'll make one just for bunnies one day!