Fallback: None. Identity servers who are not able to act on a request for this "file" will send a 404. But anyway, if we can't create such a file or rewrite the request to a valid resource we would most likely not be able to deliver the capability document with another method.
Other meta data: The relying party / identity consumer won't "see" the website located at the URL (possibly this site not even exists - bug or feature?) and therefore is not possible to extract other meta data (like a link to a FOAF/vCard file) or parse the page for interesting information for this session without an additional request. However the capability document could contain these information, too (a FOAF URL as a "service").
Filename: Which magic string to use (e.g.
yadis.xml or other) is a separate item to be discussed. It should be a filename that webservers in their default configuration send as text/xml (as the capability discovery document most likely will be XRD).
Here are some possible algorithms to obtain the capabilities URL from the identity URL:
|Draft-002-c-1||Regardless of the identity URL, append a magic string to the identity URL, such as
|Draft-002-c-2||Magic file (such as