A solution to NOT drill your track forks and have a normal front brake nonetheless

I'd say wedge.

A star nut won't have the possibility of coming loose over time like OP says a wedge has the potential to do, BUT:

1. It's a temporary brake right? So it'll be removed and re-installed frequently anyway.
   
2. As a temporary brake, a star nut makes it difficult to remove i.e. you have to take the stem off, put a pipe down your steerer and give it a whack.
   
   

What oz. said about mounting it behind is a good idea, but it's for track frames, and track frames don't have enough clearance!

If it's not a temporary brake, I'd go with star nuts on top of a wedge. I can see a star nut, even several of them, buckling due to heavy braking. A snug wedge'll stop it moving around, therefore stop any potential buckling, and with a star nut on top it'll stay put.

You could also have a U-shaped brake bridge (like you get on MTB front forks) that goes over the front wheel and mount the brake on there. Then just jubilee-clip-esque the bridge to the fork legs. Or is this how clamp-on brakes work anyway?