Horizontal dropouts with disk brakes are tricky. My design turned out not to work, and after a bit of thinking I've decided that the problems are inherent in the setup. Ideally I'd have the caliper either in front of or behind the axle rather than above it, but doing that while still getting the 12mm of horizontal adjustment I need (if I want to avoid half links) proved impossible. So I put the caliper above the axles, and hoped that the braking forces wouldn't move the axle. Which turned out not to be the case.
So I scrounged up a pair of Rohloff-compatible vertical dropouts and welded those on instead. The photos are quite poor because they were taken with my phone, and it's not too good in low light. I only thought to take photos after completing the left hand side dropout, so you can see that in the background. I don't normally grind after welding but in this case I haven't picked up the welder for over a year and decided it's easier just to spray'n'pray than spend an hour doing test pieces to get my hand it. I need really smooth flush joins around the axle inlet because bumps there make the wheel hard to get in and out. Then once I started gringing it's very easy to do the rest :)
From left to right: cuts marked out on the dropout; rought cut with the grinder; wheel in place for positioning; tacked (and look at the gaps to fill on the right!); welded up; after grinding.