My background is as design engineering, and would definatly like to help. I dont have the vehicle you guys have, so im not sure of the setup. But some things to ponder.
1st-is the nav system/display a self contained computer module? if so this may or may not be tricky. first it is an integrated board, it may be proprietary technology, ie unconventional bus, mem management, OS, etc. If this is the point, it may not even be practical(you would have to map memory, discover chipset command sets, etc, then hack low level, ie assembly, and compilers)
2nd-chances are though it's build off standard ieee standards, such as x86, risc type command sets. if this is the case then access would be a little easier, once you issolated this, as well as OS, im sure you could quickly(months?) create/access the api.
now the way i would approuch this project would be to indentify a few things. such as communication bus between the nav unit and ecu(probably mod bus or sum such), and figure out if i could use the display, display drivers independent of nav system/computer. At this point i would just use a new puter(biscuit form factor or similiar) and code my own custom display. You would need the modbus command set to interpret incomming signals.
NOTE IN FACT.
on my wd21 im designing a custom dash, and am integrating a custom computer, with touchscreen, wifi, nav, and analog/digital I/O's it's a ton of fun. hopefully ill have it done within the next year.
my purpose behind this stemed from the "what if's" and in the end i will everything will be ran through my puter, including ECU. I want complete control over throttle curves, mixtures etc, plus i want video, mp'3's, itunes and wifi, Im using NI Labview, as well as custom c/cpp Dll's im coding out.
just have to find the time to work on this more, as well as multiple other projects