Sometimes ideas just pop in. Here's one for a basic optical beacon tracker that can possibly operate over extended ranges.
An optical tube (possible zoom, definitely close to focused to "infinity") has an array of IR phototransitors/photodiodes at one end. The target has an IR beacon (IR transmitter, probably driven by a fixed rate PWM from a microcontroller). By either just tracking which transistors are on, or in a more advanced variant, how much energy each sensor "reads" from the target (or if too many see it), the direction that the assembly has to be turned can be calculated. Power levels help to determine the "zoom" of the optics (if fitted). The PWM'ed signal allows either a hardware or software filter to be constructed to look specifically for the PWM signal (similar to the carrier frequency used in IRDA)