I've been using the torque app for my Nexus 4 for a while now. Love it. The bluetooth cheapo straight from China ELM327 dongle has been a bit temperamental, but works.

Just wondering though, would it be possible to use the dongle/phone combo as a piggyback on the ECU for tuning?

Not talking about flashing the ECU, but a "simple" piggyback.

I wonder if someone can make an app for that.

Also... I've noticed that my fuel economy has gone up since I've just left the bluetooth dongle plugged in all the time (in the OBD2 port). About 2-3mpg (noticable. I calculate every time I fill up). Very odd... (maybe it's just the weather or good gas, but I'm fueling up at the same place and driving the same roads i usually drive)
It's probably just in my head...

 

As far as I know thats not possible on our cars. Even if it was unless you knew what you were doing I wouldnt bother with it.

 

No, not possible. It reads output, not upload data or maps. I love the Torque product though, it's the best app I ever bought. OBD2 was designed so we couldn't screw with the maps, which is why you chip an OBD1 ECU.

As far as gas savings, placebo effect. Your gas station probably just switched to the winter blend.