In today's procrastination at work, I managed to find the FCC ID of a PowerTap SL 2.4 hub: T8P-SL2P401. I was then able to search the FCC's public information page, and find all sorts of engineering information about the hub. The site is good for pictures of the "torque tube", so go take a look!
The 2.4 GHz internals of the hub (2457.0 MHz, to be precise to five sig figs) were provided to Saris by DynaStream, the company behind ANT technology who is owned by, surprise surprise, Garmin. The acquisition happened in December of 2006.
At this point, the incompatibility is just due to politics! Honestly, how hard is it to hack the firmware in the Garmin Edge 705? I've gotten the usual cold shoulder from Garmin when I contacted them about this. Absurd.