I am not sure, but I think you are wrong about the splined interface for cassettes being proprietary. There are so many cassette makers, that I really doubt it.

And if you are right, it still makes little difference, because so many manufacturers have the right to use it (payed the royalty already), that there should not be problems in manufacturing a fixed hub with such interface.

As for developing of play: I use singlespeed conversion on all but one of my singlespeeds. So, I use one of those splined BMX sprockets on a normal (freehub) hub, and no play ever develops. Maybe all my hubs have a steel body.