When my old Guinea Pig was suffering in the heat, I found bathing her helped her to cool down.
Obviously you can't bath a rabbit but a cool flannel wiped over their ears, if they don't mind, is also a good way, as I believe this is where they lose excess body heat anyway.
I had to do this with Feegal last year as he was looking really uncomfortable. Thankfully he didn't mind it.
If they don't like it then I wouldn't do it as the stress would make them over heat anyway.