There are strains of willow running up to 30-35% protein in the leaves, but i suspect whatever grows local will do the same. Treeleaves are more nutritious then grass/hay. But pellet for rabbit is more tricky, you often end up with high starch along with it and starch needs amylase to digest. That is more for granivores like humans and to a lesser degree chickens, but not herbivores like rabbits.
I can get a grasspellet that goes up to 16% protein and they'll grow well on (or fat).
But i prefer a week or 2 longer on more apropriate food for their digestion and healthy animals. A little patience turns out way cheaper in the long run. So sit down with the prices, rations and such and calculate what works out better for you. You may be better of on some simpler feed for a little longer then speeding up growth for higher cost per animal.