Well, with a PIE HON03/PC-SON adaptor I was able to get things working. I encountered a problem where since I have the adaptor installed in the trunk apparently it's too sensitive to the presence of voltage bleedover on the 'ACC' lead and would randomly turn off. I was able to solve this by grounding the 'ACC' lead to the Ground at the adaptor itself, and plan on using a less sensitive relay to do this only when the ACC lead is grounded by the stereo.
HU GND <-> GND ADAPTOR
GND-ON/DIS-OFF <-> +5v
If anybody knows how to make the relay circuit to connect the +5v lead to the ground line when it is grounded by the HU please let me know. I plan on researching that tomorrow.
Once I have that working, I'll have to work on some custom code to get the phatbox to power on and off properly. While playback is halted it never turns itself off completely and would end up sucking alot of power.