I broke down and ordered 4 of the rack mounting brackets today. One for each bike, one for the next n+1. My bikes don't really need one, though I've been wanting them anyway, as my seat is high enough that a seat post mount remains visible, but I need to mount a decent light on my wife's bike, and her seat is below the rack height. She also has funny oval seat stays that don't take a normal attachment, even ignoring that the light would be very low and partially blocked there.

I hadn't considered the more permanent attachment option, and my first attempt to jury a clip failed, resulting in me losing the only available mounting clip that I had. So I needed a new clip anyway, so I just broke down and ordered a bunch of the rack mounting ones. I hope they don't take long to ship. Not much night riding at this time of year, but I hate waiting for things.