I seriously enjoyed The Bells Of St John, the first episode of the second half of series 7 of the new run, but I was rather annoyed that I couldn't modify my own Wi-Fi network to further enhance the fun. It is feasible to mock up something pretty close though. The method (with a certain quantity of spoiler content, so be warned) is after the jump.
The issue is that the non-standard character set used by the aliens in question isn't straight alphanumeric in nature. It turns out (via reddit) that there are unicode equivalents that are pretty close, however. It's unlikely you'd be able to get them to stick on a regular router, but it's feasible to use them on a suitable DD-WRT enabled router, although the results may vary depending on which device you're connecting with.
I'm particularly fond of the hack enabled by Anthony Biondo, which uses a Linux fileserver and a default landing page. Most devices that connect to a new hotspot will launch the landing page, which… well, I'll let you see for yourself.