Although I have not done this myself, I have seen it done very well. A friend of mine installed a simple rocker switch inside his glovebox. The switch was inline with the ignition wire from the key switch. When he felt the car may be threatened, he flipped the switch and opened the circuit.
If I were you, I would use something like "the club" or a similar visible device. As my friend eventually found out, your car still being there is little consolation when your window, dash, and ignition switch are destroyed. Unless you have a really unique and high end car that would be stolen specifically because it is what it is (and in which case you'd also have the $$ for an alarm), a visible device is the best option. All unorganized car theft is a crime of opportunity; they'll pass you over. I heard yesterday that 1/3 of the 800 cars that were stolen in Charleston last year had the keys in them!
1982 Mercedes-Benz 300CD
1982 Mercedes-Benz 240D - stick