Sometimes, if your fork isn't too bulky, there's a cheap and easy work-around.

Use the front caliper on the back. That lets you use an exterior nut to secure the caliper.
Use an extra long recessed nut to secure the rear brake caliper on the front. New brake sets commonly come with varying length recessed nuts so, if you can find one that works, you're good.

Drilling out the front hole rear brake bridge isn't as hard to do as one might think. You can't get a drill in there but I've grabbed the drill bit with my vice grips and rotated it by hand to drill out the bridge. Only takes a couple of minutes.