AutoZone and the like sell little led lights that are on the end of bolts. I believe they are made for license plates. Take the license plate bracket and light off from your fender, cut the wires and then splice them onto the "bolt lights". You'd just need to determine the best way to run the wires to avoid the wheel and belt.
Depending on what route you go with where you mount the plate at would determine where you need to run the wires.
This isn't the best shot, but it'll give you an idea:
I removed the bracket, light, etc and relocated my plate to the vertical mount off the axle nut. I did NOT put a light on my plate yet, but need to get that taken care of ASAP.
Although - my plan is to remove the seat and go with a solo seat and trailer fender... You could take the tail light off from where it sits now and PROBABLY mount it from under the fender some how?
I think my plan is to go with a set of red turn signals that will double as my brake lights. There's a few bikes in my town that have that setup (I believe it's stock from HD that way) and it looks sweet.