I asked a similar question around Feb 18, 2003. Francois Lachance had a
valuable response in which he suggested something like your solution #2:

       <classDecl>
         <taxonomy id="general">
           <bibl>
             <title>General Taxonomy</title>
             <ref>http://www.foo.org/taxonomies/general/general.xml</ref>
           </bibl>
         </taxonomy>
       </classDecl>..

I had been thinking about using xptr instead of ref:

       <classDecl>
         <taxonomy id="general">
           <bibl>
             <title>General Taxonomy</title>
             <xptr url="http://www.foo.org/taxonomies/general/general.xml" />
           </bibl>
         </taxonomy>
       </classDecl>

I noted that adding a "url" attribute to xptr requires a statement in the
internal subset, but someone else pointed out that the Guidelines do
suggest that we do this.

j