Instead of the Topeak Tourist I went for the Madison Ridge rack.
It's 200-300g lighter. Can carry up to 25kg. And you can get it online for under £20 if you google around.
It uses a bolt-through-a-tube system too. Since buying mine, I've realised I may not have needed a disc compatible rack despite have disc brakes.
Have a look at this picture (
http://www.flickr.com/photos/bealer/567 ... hotostream). Basically the upright part of the frame fits (and I've tried perfectly next to the caliper if I use only two washers so it doesn't bump the upper part of the caliper. There's a gap about a hairline wide between the vertical and the circular brake pivot. Unfortunately the Madison Ridge rack can't be "squeezed" as it'll strain the weld points.
I don't think the bolt/tubes will be an issue. What I don't like is that my panniers stick a couple of inches further out on each side than they would with a non-disc rack. Going through cities I like to be nice and narrow. Plus out in windy places it makes me less aerodynamic. Despite that, for the weight and cost I've not been able to find a better rack.
So it might be worth taking your bike to a few stores. Ask them to see what fits. Obviously certain designs won't. You need a design where the diagonal support is attached to the vertical one that will allow enough space for the caliper.