Gentoo should have an official off-topic IRC channel. It looks like #gentoo-ot on Freenode is registered by a Gentoo dev (dmwaters@gentoo.org). I think that this should become the official off-topic channel for Gentoo, and be listed on the website. That way on-topic people have a place to send conversations when they become terribly off-topic. Having it listed on the website would make people much more likely to go there on their own, in search of conversation with other similar-minded people.
I seriously doubt adding #gentoo-ot to irc.xml is going to diminish the amount of OT discussion in #gentoo and other channels by any significant degree. That said, added it anyway. Just for giggles.