I was doing some searching for a way to add brake lights to a 300ex and 400ex. I stumbled across these little hydralic brake switches, which simply replace a banjo bolt anywhere in the system...coming out the master cylinder of either or both front and back. Add some simple wiring, and pick up a cheap tail light off of ebay, which has both tail light and brake light wires, and I now have brake lights.
The thread size on the hondas are the m10x1.25, for the banjo bolt.
one of the places Ive found them

 

I did this to my 250R and it works great. I bought a tail/brake lite for a 450R on Ebay and it bolted up perfectly. I put two switches on mine, one for the front and one for the rear. Parts Unlimited also sells the switches if you are looking for another place to get them.