Fallback ("unwanted"): Implementation of Draft-002-a and -b will automatically accept a direct link to a capability URL (which will always send the capabilites document independent from header or query) as proposed in Draft-002-f.
Fallback (possible): Users who can not deliver different content based upon the header or query will not fail on a request but simply deliver the standard HTML page. Then Draft-002-d can be used as a fallback alternative (as already in Yadis spec).
Other meta data: The relying party / identity consumer won't "see" the website located at the URL 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 this information, too (a FOAF URL as a "service").