I think the problem is that OBD II has both generic and manufacturer specific codes. Any old reader can handle the generic ones, but you need software for the particular car model (or at least the manufacturer) to do the specific stuff. So firstly you'll need a CAN bus to serial converter, then you'll need software that's compatible with both the converter and the specific vehicle to decode all of the interesting stuff.
On the subject of CAN bus, these look like good fun, but their
shop seems to have ceased trading:
I reckon you could do some interesting things with one of these.
I'm also wondering if a standard Arduino could be used as a fully programmable digital chip tuning box. The analogue input should be capable of reading the fuel rail sensor, though you'd probably need to convert one of the PWM outputs into a true analogue voltage. With this arrangement plus software, it should be possible to return a reduced fuel pressure reading most of the time without letting it slip so low that the
registers an error (I'm convinced that this is the problem with regular tuning boxes on the dCi 130.)