If you wanted to be really cheesy, you could build an adapter like I made for moving the canti posts for a 26" frame to 700c wheels.
The difference between what I did and you will have to do is that you will have to use some sort of strong hose clamp to attach the horseshoe to the frame. I used the existing 26" brake studs which you don't have. Using strong hose clamps really shouldn't be a problem since the majority of the stress from braking hard will be exerted on the horseshoe itself rather than the clamps.
This is what I made:
