Melvang, I would imagine they use a common ground (alaric's test is further evidence). As far as I can tell, there's no reason not to. Keep a few things in mind, though.
1. If you use any sort of TRS connector for this, both the tip and the sleeve will be shorted to ground when you insert or remove it. So, if you trip over the wire, unplug it, etc. it'll activate power and reset switches. Normally just tapping the power button triggers an ACPI event when the computer is on (usually brings up a menu asking if you want to power down) but this may not be ideal behaviour. I recommend a different connector for this.
Otherwise, you should be able to do this with relative ease.