That's not technically a valet switch, but yes it would work with the proper protection. Problem is though, there are multiple ACC lines on a car, all connected to the original starter. It's hard to explain but you'd have to power them all to keep the ACC line hot.
If you JUST wanted your computer hot, you could just send +12v to the ACC line of the PSU you're using. Except be careful in case you forget to shut it off. Make sure you add some sort of 555 timer that starts when the ACC goes off.