It depends on how you want to use the beacon. If you are just trying to determine the position of your alliance partner, relative to your own robot, you could construct a system which would be within the rules.
You could mount a camera on your robot, and look for a unique feature that you place on another robot. Think of the florescent targets on the
2009 robots. Or train your camera on a unique
fiducial marker that you've trained your camera to detect.
If you're trying to send information to an alliance partner (inter-robot communication), I con'at think of a way to legally implement this that would be reliable enough to use in competition.