Ive looked into hay only diets and tried it with Ivor, it seemed to have a really good effect on his poos and he lost alot of his christmas weight.
I would have kept it up but Anna came along and she is very thin-whatever Anna eats Ivor eats, having them on different diets and living together is impossible.
In nature rabbits rely on grass but they would sometimes find some nice veggies, I would say 95% hay 4.5% veg and .5% pellets is a good diet, most people would recommend a much higher level of fresh veggies.