I designed and manufactured my own disc brake tab for my horizontal dropout Surly 1x1. Here it is mounted in an Anvil jig. Will this work? Am I missing something? I put the dummy axle about half way in the dropout. The caliper can pivot up out of the way to remove the wheel. The tab is not fitted yet, so that's why you see a gap between it and the seat stay. It is also not the final version. I still have a little tweaking to do.
http://farm4.static.flickr.com/3068/2694112066_e0fbc274d7.jpg?v=0

I can't see a problem. The big issue is would the rotating force want to move the brake in your slot. That might not be a dela breaker, but having the posibility of the brake spitting out isn't good. I'm having trouble visualizing how that would happen, seems like it would load a little perp. to that. What do you think if you mock up the brake and the wheel?

The other issue is that the wheel can be pried out, I don't see that either.