i haven't looked at the latest patch yet, but it occured to me on the train this morning that a better mechanism then matchall=
would be altq=
with the psuedoe code being...
Query mainUserQuery = null;
if ( params.get(Q) is not empty )
mainUserQuery = dismaxParse( params.get(Q) )
else if ( params.get(ALTQ) is not empty )
mainUserQuery = SolrQueryParser.parse( params.get(ALTQ) );
if ( null == mainUserQuery)
...that way people that want a MatchAllDocs fallback can put altq=: in their defaults or UIR ... but if they want it with a different boost (because they are using BF and BQ) they can do that, or if their index is heavily partitioned by doctype and they only want certain types of docs to be included in the set when no Q is specified they can use "altq=doctype:(product blog)"