hook_form_alter on search form to add facets before search

Hi,

i've seen a nice presentation here:
http://www.archive.org/details/drupalconchi_day2_attain_apache_solr_codi...

i'm particularly interested in the case 'I want initial searches to be targeted.'
(on Drupal 6)

in my case, the dropdown lists content types (same as 'Filter by type' facet)

i would like to pre-filter search with these content types; default is no filter

once the search results are displayed, the content type selected via the dropdown should be already selected in the facet block filter but still deselectable (as on a normal faceted search)

i also had a look on this thread: http://drupal.org/node/443986
all without success

i hope someone more experienced on Solr in Drupal can help me setting it up

thanks a lot