Just had a quick brainstorm re: rack mounting. My 4-A rack ends address the bottom of the rack, but the top of the rack is equally challenging. One common way to attach a rack is through the brake hole on the frame/fork. I would always turn down the head of a M6 SHCS so that it could be inserted into the rack tubing; I'd then use a standard recessed brake nut on the backside. I made a quick & dirty sketch:

It might be useful to have these parts pre-made, so that rack builders don't need to turn their own on a lathe. If you'd be interested in a part like this, holler at me!