Here's an idea: when a user ask for an Hash Index transparently build a BTree index over an hash function instead.
Could something be added to the planner so that you can just build a btree index on a hash expression, and have the planner automatically realize it can used for an equality query because if a=b, then md5(a)=md5(b), at least for some data types? A general ability to recognize things like that seems like it might have other uses as well.