Clamp-on brake for undrilled forks?

I don't know about the brake but I wouldn't ride that gear all the way there - you'll be knackered! I'd get a flip-flop rear wheel if I was you (you could always put a stupidly spinny gear on the other side and fixxie-skid your way down, save on brakes!)

only possible when you have either realllly long trackends or bring a chain with you