Unfortunately, the Guest Network on the AirPorts only offers 3 security options:
So it is not possible to configure the Guest network to use WEP encyption.
A possible workaround might be to add an older used AirPort Express and configure it to provide a separate network using WEP encryption.
Another option would be to add a new AirPort Express and configure it to "extend" the wireless network. Then connect the old computer to the Express using an Ethernet cable and turn off wireless on the computer.
The possible advantage to doing it this way is that you will have more wireless network coverage with the addition of the Express, so other wireless devices should pick up a stronger signal in the area where the Express is located.
The article is talking about options for the "main" network.
The "Guest" network feature has never had an option to use WEP, so the article....while interesting....would not apply to your Guest Network question.
In any case, WEP is long out of date and easily cracked by any number of free utilities available on the Internet.
Even if you could enable WEP for the Guest Network, any teenage kid with a laptop would probably already have the tools on his comuter to crack the network in a few seconds. Unfortunately, there are plenty of sites that kids visit that tell them how to do this.
Any way to connect an Ethernet cable from your router to the computer? That would be the "best" way to do this.