It looks like you'll also need to add a condition to identify the "weather" feed type (around line 217 in my copy of that file) where it's checking for RSS, RDF, and Atom. The changes might get a bit more extensive after that since it probably uses those values. If I get a chance I'll take a peek at it.
I wrote something similar recently to pull weather data
from NOAA's XML service, but I used GetXMLTree and some of the other functions to do it since the weather data isn't very RSS-like.