filter more like this results by the site making the request

hi, i'm running a large multisite solr implementation. i am wondering what is the best way to refine the results in the more like this block to only return results from the site that made the request. if it was a normal search i would alter the query and add a filter for site, but there is no query object (as far as im aware) in the search:

$response = $solr->search('nid:' . $nid, 0, 10, $params);

is there a param that can be added specific to mlt? after looking at the ver6 module i had a go at declaring the query as a query object and then modifying it but had no luck.

at the moment i am getting what would be relevant results but from my other sites which either end up linking to the wrong node or just 404ing to node/xxx

any help would be greatly appreciated.

thanks

joe