I also thought about brake light on top and turn signal on lower (rectangle tail lights) but seems to me that you would have to cut the reflective lens out and replace it.
I know that are some newer CLEAR 3rd brake light license plate frames using LEDs.
Some interesting LED frames and light bars:
http://www.rodworx.com/products.php?scatid=4
http://www.customdynamics.com/Light_bars.htm
If the christmas tree lights around the rollbar don't work ....
how about a wireless 3rd brake light on the back of your baseball cap?
http://www.webbikeworld.com/r2/riderlight/