..... i don't know how the CVN system works, but every GM OBD2 program that i've disassembled DOES have an option to ignore the calculated checksum... meaning you can leave the checksum at what the factory program is and change whatever you want, just have to have the option enabled.

if the reading device reads the PROM checksum, then this is another workaround. if it is able to request that the ECM perform a checksum test and report back it's results, that could be an issue....

however, i've also seen a LOT of free space in most OBD2 calibrations.... if the tuner were setup to do so, it wouldn't be very difficult to add values into blank areas of the PROM to get the original and new checksums to match. now there's really no method to determine a GM tune from a custom one without pulling the entire BIN and comparing it to a known GM BIN. i don't see this happening, even at a dealer.