I'm trying to set a transfer cap

I have two rental units in my house, and provide free wifi for guests. I have a 120GB monthly transfer cap, so I'd like to limit the guests to 1GB per day, each. Some guests (particularly in the 2 bedroom unit) bring multiple wifi devices, and they all need to be included in the cap.

A friend suggested WifiDog, which I've installed, but it does not seem to do what I want, or not very easily.

I've read around a little more, and have a new idea - please could you let me know if I'm thinking on the right lines?

I now plan to set up two virtual WLANs using DD-WRT, and use iptables quotas to limit the transfer. I have a Linksys WRT54G that's dedicated to this project (our home wireless network is on a different AP). Does that sound like it will work?

Bonus: my son would get his Minecraft server back

Thanks for your thoughts,

Rupert