I tried to use some Sweet Feed for general foraging livestock. It has 12% crude protein, and it is recommended that rabbits have 16%. The sweet feed worked fine, and the rabbits did get spoiled on the molasses. I used approximately a tablespoon of Calf Manna with 25% crude protein to supplement my adults each day. Now, I will also say, I feed pellets in the morning, and all the alfalfa-clover blend hay they want to eat in the evening. My fryers still made it to 5Lbs in 10 weeks. My Jr Does made it to 10-12 Lbs easy enough by 8 months, and my Jr Buck reached 10 Lbs by 6 months. Now I realize this is not "natural" feeding to avoid the feed stores, but I was paying between $6-$8 on the Sweet Feed. The problem I faced was that the two particular Farm stores I frequent kept changing types, brands, prices, making it hard to stay reliable with a set bag to buy as needed. I now use a rabbit feed formulated for rabbits that runs $12.
There is several plants and weeds you can research that the rabbits love to eat and is healthy and natural. I have read of some going completely natural with things they plant and forage. They use a mineral/salt block too, but you may end up guessing on the required vitamins and nutrients needed. I am interested to see what you find out and what you choose to do.