Feeding grain will not harm a rabbit.
I gave grain to rabbits for years not one had a health problem..
If you look in the book Storey's Guide to Raising Rabbits ( author Bob Bennet )there is a all grain mix that the former president of the American Rabbit Breeders Association Oren Reynolds had shared with Bob Bennett.
6 quarts oats
1 quart wheat
1 quart sunflower seeds
1 quart barley
1 quart kaffir corn (when available)
Add 1 quart Terramycin to mixture once a week.
Feed one parts of this mixture to three parts rabbit pellets.
I use the mixture and I like it.. I feed 3/4 cups rabbit pellets in the morning and 1/4 cup grain mix at night.
Chris