How are you at desoldering and soldering? I just looked at mine, and it looks like it would be relatively simple to desolder the existing switch from the board. Then you could drill a hole in the back case, run a wire through it, and solder the wire to the contacts where the switch was. You would need to remove the plastic lens from the part that holds the batteries and board to access the board for this procedure. It's probably glued or fused on somehow - probably won't just pop out and back in. Then run wire to the handlebar and solder to the original switch. You'll need some kind of small (preferably waterproof) enclosure for the switch to fix it to the handlebars and protect it.

If you could find a suitable switch, maybe it would work to leave the original switch in place and solder the new wire alongside to the existing contacts. Not sure if it would be a problem if both switches are left in the on position. It might cause a short-circuit that would cook your batteries or something.

These are relatively cheap lights. You could try it and report back. Or buy one of the $5 knock-offs and play with that instead....