I think I've said this before, but life is *much* easier when you have an engineering level contact at the vendor.
After my previous post, I had an extended discussion with Steven Toth (who committed the original GPIO fix) on strategies for determining the correct GPIO to reset the xc5000. These included ideas like adding code to poll the 24 GPIOs in succession to and waiting for the tuner to become unresponsive. I was also considering removing the RF shield from my device and tracing it out by hand with a meter, which is pretty laborious since I don't an SMT rework station.
In the end though, the developer from PCTV Systems got back to me overnight, and just told me what the correct GPIO was. This eliminates any need to reverse engineer the reset pin entirely!
I'll be testing it on Sunday. If you're trying out the xc5000-improvements branch, expect the Pinnacle 800i to not work properly until then.