Ok, I've done some searching and unfortunately, there are no 2 byte prefixes for all 3 possible lengths if we increase everything by 1 or 2 bytes.
I'm down for increasing the length a bit with a random salt. I don't have any strong opinions on how this is done, but it sounds good to me.
That would also mean we could go back to my original idea about sha(sha(privkey)) being used as a bloom filter element.