I've heard of the Ravelco as well, but I'm hesitant to have it put in my hobby car because if I want to change something, I'd be in fear of messing somthing up.
Currently in the Malibu I hid the brain somewhere besides the typical under-dash location, and also while it's armed there is a hidden siren inside, so the second they turn the ignition on the siren starts going off.
Another option would be setting a latching relay as an automatic kill switch:
http://www.eleinmec.com/article.asp?24
Basically you wouldn't even worry about the reset button in this circuit (unless you wanted a kill switch to shut the ignition off while the car is running). You could then wire the trigger button as any momentary switch in the car (use your imagination here... what uses a momentary switch in your car?), and the Vs side of the circuit would be on the ignition switch side, while the device portion would basically be the other side of the wire from the ignition switch. This way, as soon as you turn the ignition off, power to whatever you have this relay connected to is killed until you a)turn the ignition back on, and b)hit your hidden trigger switch.
This way, with your hidden energize switch, a would-be thief would be none the wiser to what's needed to start the car. I'm planning to do it, you should too
😉