Yea I've thought of ideas similar to that back when I had my corolla going. I wanted to throw a small computer in the car with a GPS and wifi antenna so I could track elevation (up hill vs down hill) and also track the fuel usage, TPS reading, gear, temps, etc. The idea was to collect enough data just driving the car where it could predict the best route based on my driving habbits, and also it should be possible to have it suggest better ways to drive. Clearly coasting more is always a plus, but maybe just minor throttle changes on hills and such would be a help.
Since the prius is DBW, it's a lot easier to electrically control the throttle. With that control also comes a massive liability if I ever sold anything like that. An open source project might be interesting though. Tapping into the gear select switches and pedal shouldn't be too hard. ODB2 could get most of the other data needed, but reading directly off injector #1 I think would be more accurate for fuel usage.
I've been building harnesses for derby cars lately, they want to delete as much wiring as possible, remove protections for the engine, and all of that kind of stuff. That's why I've been looking into the micro controller stuff a fair bit. A speeduino should be able to control the Prius engine no problem, but it doesn't support the DBW pedal and throttle plate control. If those issues were solved, you could have more control over exactly when the engine started, and how it responded to throttle inputs. The hybrid computer will still control the regen and such if I'm understanding the wiring right. I've looked a little at it since there's a couple people out there want that to try to derby a prius. I suspect the car would hold up well in a derby, but the electronics up front would probably need to be relocated inside the car and then there's high voltage wires to deal with and such. Not exactly a friendly car to build for derby use lol.
The stock cruise control switch should be possible to be used for the automated pulse and glide computer, but I'm thinking it won't be possible to be able to switch between them too easily besides adding like a toggle switch somewhere. Clearly would need wiring tied into the brake pedal and such so it can be canceled, and all of the basic safety concerns solved. I'm fairly new to hardware designing, but I've programmed most of my life and get the basics of hardware design well enough to have an idea of what's going on at the circuit board level. This might be a good little project for around winter time when business is slow.
The board with all the lights is to show injector control and ignition control and such. The screen is the data from the speeduino, TPS % and such. Everything there is simulated, but there's quite a few people running engines off that project. It seems to be common over in the EU and AU more in the racing scenes. It might not be legal to run a car under 25 years old on a speeduino if it's for on road use (EPA/DOT regulations). Not sure how likely someone would get caught in a state with no inspection, or if it's like removing the catalytic converter where most people don't care. My state doesn't have inspection, for anything older probably 50% of the cars are missing the cat since the scrap prices for them have been crazy.