I've attached some photos of my solution below. Click on the images to make them larger. As you can see in the first photo, the rack is not very level, but it should be good enough. I think it would be more level with a 180 or 185mm rotor, but I decided not to make that compromise. The attachement method at the dropout is different on each side.

On the non-drive / disc side, I've used the upper bolt hole on the rack and I've bolted it onto the lower disc brake mounting hole (only the fender is attached to the normal fender/rack mount hole). The bolt for the disc brake mount is an M6, but the upper hole on the rack is an M5 size (only the lower hole is M6), so I had to drill out the upper hole on the rack so that an M6 would fit through, but that was easy enough. I still had to space the rack out a little to make it clear the disc brake, especially to clear the cable housing entry point. I therefore used some spacers between the rack and the frame, I believe there is about 10mm of spacers, which is the most that I would want to use, but I could not use less. These spacers came with the Tubus rack extension kit shown below (see product page here). I did have to find a longer M6 bolt to secure everything with, and the one I did find was actually slightly too long, and touched the disc rotor, so I had to put another small spacer behing the bolt's head. In the future, I may trim this bolt and get rid of this spacer.

On the drive side, I had the task of making a mounting point similar to the other side so that the rack would be straight. This is where one of the metal plates from the extension kits came in handy. When I bolted this extension plate onto the rack in the way shown in the fifth photo and then clamped the bottom hole of the plate with the quick-release skewer, it was in just the right spot. Tubus sell a very similar kit that is designed to be bolted on using a QR, so I assume this will not be an issue. The bolts used to connect the extension kit to the rack would have pushed up against the frame, so I had to put a small spacer behind the extension, also on the QR skewer, this one is probably only 3-4 mm wide.

It's certainly not ideal (I'd like the top of the rack to be flat and I'd like to not need to use any spacers), but I'm pretty sure that it's the best I can do with this combination of frame, rack, and brake.

BTW, yes I know the bike and chain need a good cleaning, that is the next job on the list. And the frame is really not as pink as it appears in some of these shots, it's just a bright red.