Jesse, you're on to essentially what I am doing, however it is for a side project of my own. I do have access to all the CAN database information, so no reverse engineering for me. I just need a method of filtering and modifying signals on the network between multiple devices. I know the commercial tools that are out there, but they are obviously beyond my budget and I can't exactly "borrow" a dSpace MicroAutoBox and Simulink license without getting a bunch of questions.
I'm not a programmer, so I need to find out how good some of the open source stuff is. I see guys using the Arduino for scan gauges, scan tools, and sending messages on the bus (but these are typically on/off switches, like windows etc.)
I need to find more info about OpenCAN and what off-the-shelf support there is for CAN with Arduino. From everything I have read, it should be possible. But the GUI's may be lacking in some areas.
As I learn more I will keep this updated for people who would like to do things similar. However I will continue to be vague in the specifics of my implementation.