Does your car have a factory alarm system right now? I would have to look into it to figure out how to wire up a shock DEI shock sensor to the factory alarm. . But the basic function of the DEI shock sensor is simple. It has 4 wires on the harness; Red, Black, Blue, Green. Red gets constant 12v +. Black is your constant ground. Green and blue are your negative trigger outputs. One is a warn away, and one is the full trigger.
So when someone bumps or physically tampers with your vehicle, the shock sensor translates the vibrations into negative pulse outputs. Well you have to have something that receives those negative pulse outputs (an alarm) and uses them to activate a siren or sometype of alerting device (lights, horn, 2-way pager, etc...). If the BMW factory alarm has trigger inputs like aftermarket alarms do, then we most likely can tie the sensor into the factory system.
If you don't have a factory alarm, we could hook up a shock sensor to a siren or horn but it would not operate in a very desirable manner. It would require the use of a relay. But the output from the shock sensor is very brief, so the siren would only chirp. Your best bet would be a true alarm system. I'm not sure how the user you read about tied his shock sensor to his hood pin. The hood pin is a trigger source, just like the shock sensor... The only way I see them existing in the same cirtcuit is if they are feeding into an instant trigger wire and being diode isolated from one another, but again, the instant trigger wire is something you find on an alarm system.