Before installing my Alpine w505, I researched the hell out of this. I also had the install shop research this a whole lot as well. They work closely with PAC Audio on alot of installs, and they simply were not able to come up with anything.
Sorry to say, as far as I know, there is no way to maintain all the functions of the OBC. It still computes Range, and MPG though. I still found the advantages of the W505, to outweigh the loss of functions that the OBC provides...