Unable to make solr query work with facet filtering & count

Hello,

I am trying to solve exactly this http://drupal.org/node/1446824#comment-7038662, but in D6.26.

Taking a specific query as a hard-coded example, I am able to get filtered results with proper facet count with the following query to the search engine:

http://localhost:8983/solr/select?q=desk&fq={!tag=dt}ss_MT:Other&facet=on&facet.field={!ex=dt}ss_MT

I naively tried everything in hook_apachesolr_modify_query(&$query, &$params, $caller), hard-coding variations of

$params['facet.field'][] = "{!ex=dt}ss_MT";
$params['facet']='on';
$params['fq'][] = '{!tag=dt}ss_MT:Other';

But it never filters correctly i.e. it yields results for all facet values, not just for the value "Other" (facet name is ss_MT).

Help!

thx

Sebastien