Well, actually OBDII is a "standard". Specified right down to the electrical and physical.
ALL cars (in the US) after 1996 have to support it.
For example the physical port you plug into is standard, it must be powered all the time (car on or off), etc.
The bus is specified and I believe it can be over either the high or low speed CAN bus?
The protocol used is standardized. Modules have node addresses and (for the OBDII protocol there is a standard message format and standard set of parameters that can be polled for from the OBDII diagnostics port. Things like RPM, speed, coolant temp, etc, etc, (in particular anything that might remotely have to do with emissions and more). Part of the impetus behind that is to support periodic emissions checks and the like. Your scanner/diagnostic tool only needs to support the standard and you can plug it into ANY car.
The protocol defines a set of "mandatory" messages and parameters and also provides for optional user/proprietary messages and parameters.
So there is the standard set of PIDs (parameter IDs) that can be polled for and must be supported, and then there may be manufacturer specific parameters that can be polled for.
There are something like 100? or so mandatory parameters and then, for my car, Chrysler has maybe that many or more Chrysler specific PIDs. Some just a variation on the standard ones and some completely unique.
So... it would be ambitious to support manufacturer specific PIDs and there would be a significant licensing fee to support that. Not likely they are even considering that.
But the standard OBDII diagnostics are, well, standard. Shouldn't be a problem there. One set would support the entire US market.
Some detail here:
http://en.wikipedia.org/wiki/OBD-II_PIDs