It's a bit embarrassing letting people see things while they're in progress, but just in case anyone is curious I've put my modified version of OBDuino32K up on GitHub. As I mentioned previously my approach with this is to try to keep it behaving exactly the same for standard 328P builds, while enabling all the extra features for a Mega at compile time by setting some flags. I've also separated out new subsystems into separate files for clarity so I don't pollute the original codebase too much.
jonoxer's obduino32KMega at master - GitHub
Right now it builds on a Mega with different pin assignments to allow the extra subsystems to operate, and the extra subsystems compile cleanly, but they're not activated in the main loop yet (except the command-line interface for managing the OBDuino from a serial console which does partly work). To use it you'd need a bunch of information about pin assignments etc that aren't documented yet but that will all follow. I only have a couple of days left to finish the first draft for the book so stuff should progress at breakneck speed.
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Practical Arduino