I don't see why this shouldn't work but I can see a few issues to keep in mind.
1. Since you're stealing power from one of the plugs you may miss when the switch is on. (or maybe even when it's off)
2. If you have good spark it may jump across the contacts of the switch when it's off.
3. You should make sure that you use well insulated wire from the plug to the switch and to the plug in the exhaust. (ie, spark plug quality) otherwise you will get some loss anywhere the wire comes near the frame or any metal parts. (causing a miss on that cylinder)
4. Probably won't have a lot of luck listening to the radio. (rfi noise) Keep the wire from any sensitive electronics. (zap!)
BTW I'm not trying to discourage anyone from trying, but want to point out that if you just use stereo wire it probably won't work.
That said, another idea that might work is if you have an extra coil mount it right near where the plug in the tail pipe is and wire it in parrellel with the coil under the hood, you can use low voltage wire to the coil and it might work better. The problem with this is that with a points ignition you're drawing twice the current through the points and will probably cause some problems and this idea probably wouldn't work with an HEI system. Matter a fact it might be a bad idea to try.
Disclaimer: Never tried any of the above, no idea if it'll work, probably would be best to ignore my ramblings.
Ron