Does anyone know the format of the data output by the PCM to the gauges?

I am going to be putting an LS1 (of some kind) in my BMW later this year and I really want to get the stock gauges working, so I need to translate the J1850 or CAN messages from the PCM into CAN messages that the BMW gauges can read. I know how to format the data on the output, but I don't have a running car with an LS1, so I have no way to collect the data I need on the GM side. Has anyone ever done anything like this? Anyone interested in doing some datalogging on their car for me? I could send you the hardware or just software if you already have an AVT. Thanks, Matt