Personally I wouldn’t give horse feed to a rabbit. Feed is designed based on the needs of specific animals and I can’t think of any reason why they should be interchanged, even with supplements. Horses can eat whole corn and sweet feed. Bunnies shouldn’t be fed these things so there are clearly some differences along with some similarities.
I have heard of people making hay tea for sick bunnies, but usually people use something like Critical Care or pellets (which is usually mainly hay anyway) mixed with water/liquid to make a mash type product. I also wouldn’t do something like this for bunnies just as a treat. The main purpose of hay is to give them fiber and to wear their teeth down. Dry hay is great for both of these things. People also give bran mash to horses and I wouldn’t give this to rabbits either.
Why would it be easier to research and buy the correct supplements to give the bunny instead of just buying the correct food?