[lemiant]

Your explanations of the possible problems make a diode implausible, but I feel it's nothing a little more complicated circuitry couldn't handle. The highly capable EEs who designed the jaguar should be able to come up with solutions better than a couple of kids who aren't yet our of college. For alternatives you could use Michael's solution or:

The diode wouldn't have to affect all the power, just the power going to the control circuitry. If the jaguar's internal computer didn't turn on, it would never send any signals to the internal relay (or whatever they use) and nothing bad would happen. The computing uses a lot less amperage so this is totally feasible.