Thanks for the response, sorry to hear about your tornado damage, I've been through two hurricanes in the last month, so I can relate, fortunately we had little damage. I'll look at the CAN-hitch also and decide which would work best for me. I planned on everything being under the dash, that's where the EPS module and motor already are mounted.
As to the other points you raise, I've already installed a 130 amp modern alternator on the car. I have an oil cooler with electric fan, and modern stereo setup, newer H4 headlights, and eventually plan on Megasquirt EFI, so the alternator was upgraded with this in mind.
I have thought of sensing the vehicle speed, I can actually buy a new rear pinion yoke with a tone ring and mount a variable reluctance speed sensor to the axle housing. They sell those parts already for the Ford 9" axle since lots of racers can use them for datalogging, that would simplify things on my end as it would be less fabrication. I have a tachometer output on my current MSD ignition box, and it is running an aftermarket tach. I could sense engine RPM with that. Could anyone point me in the direction to look to read those inputs and convert them to CAN-bus messages? Is that something that the photon processor already does? Sorry about the questions but this is rather new to me, I am an auto technician, and have a pretty good understanding of Can-bus due to diagnostics, but this programming and hacking stuff is new to me.
I was going to add the ODB2 port anyway as I can use my Snap-On scan tool at work to diagnose the EPS if something were to go wrong with the power steering, I'd just have to tell it Im working on a 08 Prius when I plug it in. That would let me pull dtc codes and see the live data that the EPS is putting out such as the torque sensor voltages. I should also be able to recalibrate the EPS such as recentering the torque sensors, etc. But if the CAN-hitch would let me connect also, I'll probably run it as it could be in a dedicated project box leaving the obd port open for diagnostics without removing it. Then I could possibly read the datastream it was putting out too for diagnostics.
And finally, yes I can still steer the car with the key off sitting still, so I should be able to get it off the road as it is definately easier to steer if it is rolling if there is a EPS failure. The car drives pretty good with the EPS in fail-safe, but I'd like to get a little more power from it, and this looks like the way to do that. On the other forum I mentioned on my first post, a guy got his Yaris EPS to quit throwing an missing can-bus code (and therefore going into failsafe) by simply broadcasting an RPM signal over can bus with a different module similar to the carloop stuff.