You read my mind, sort of
I am experimenting with a "simplified" iso interface since I can't find the freescale chip in the usual haunts. Anyway, the test bed is a duino and a piece of protoboard with a couple resistors/transistors all hotglued to the back of a 4x20. After I hack some of the obduino screens to display all the pids I want on the 4x20, I was going to just throw it on the dash next to the mpguino and have two displays
Havent got it working yet, but with the guino handling distance and fuel, I'll have a whole 4x20 with access to all the pids to display anything else, w/source code control.
It is "possible" to make a hybrid, but it is a complete rewrite and probably a larger microcontroller.
Bigger LCD is quite possible, but needs custom programming, don't complain if it doesn't work. Bigger LCD + another LCD = even better
multiple 02 sensors is probably vehicle dependant, research the obd details of your vehicle.