This was discussed a while ago and we came to no conclusion as to whether it would work on a flight without the passenger on the manifest. We did determine that, at least at that time, you could log multiple devices into one account on the same plane.

It is entirely possible security has been enhanced in the mean time.

I have not heard of any retaliation for misuse of the wifi (if it does work) but you run a risk of Southwest stripping you of your status and your points if discovered.