That is why we have a WebDAV working group. Both the 300 and 406 response bodies were left unspecified because the intention was that they be specified by a group that actually had time to study the problem in detail and come up with a [hopefully] better solution than some off-the-cuff invention of mine. It was one of the WebDAV to-do items, last time I checked.
Has this ever been done? Maybe it is time to create some standard 300 and 406 response media type?
UPDATE: Umm, should have searched a bit more I guess: RFC2295. What is left is to tell clients to look for Alternates headers in 300 and 406 responses.