Ah, I think I know where I got it from. :-)
http://blog.miconda.eu/2009/12/best-of-new-in-kamailio-300-3-route.html
So the question is, is this a "sanctioned" approach, or just something eccentric that happens to be supported? And if the former, perhaps a documentation update is in order?
I would happily contribute, I just don't know how to update the core cookbook.