howto alter submitted filter values in exposed multiselect views filters

further investigations:

currently I'm trying on views_query_alter().

$query->add_where() seems be be really close, but add_where() ads a query with an AND operator.

As I want to add values to the list, I need something like add_where() with an OR operator or
something liken $query->add_where_args().

Any idea?

Thanks for the answer.

Is there anything to read about howto to alter views_exposed_form?
I didn't recognize it in D5 by now...

After several successless tries using hook_form_alter() to inject additional values to filter for, I decided to introduce a new hook in views.module views_build_view(), which is working for me as expected and gives a clean and easy way to alter the filter values sent from exposed filters without actually breaking the query itself.

Szenario:
I need to filter content types for taxonomy term ids, which are part of a very huge taxonomy term tree structure (about 11000 terms).
I created a filter widget with a checkbox which allows the user to include all child terms of the selected terms into the filter function.
Depending on how high the selected root term is located in the taxonomy tree, there is a high number of child term ids to be inlcuded into the filter values (about 5000).

function views_build_view($type, &$view, $args = array(), $use_pager = false, $limit = 0, $page = 0, $offset = 0, $filters = NULL) {
  views_load_cache();

  // Fix a number of annoying whines when NULL is passed in..
  if ($args == NULL) {
    $args = array();
  }

  // if no filter values are passed in, get them from the $_GET array
  if ($filters == NULL) {
    $filters = views_get_filter_values();
  }

  // *************** hook_views_filters_alter() **************************
  // Call a hook that'll let modules modify the passed filter
  foreach (module_implements('views_filters_alter') as $module) {
    $function = $module .'_views_filters_alter';
    $filters = $function($view, $filters);
  }
  // *************** hook_views_filters_alter() **************************

  views_set_current_view($view);
  ...

which can be called like

function cdm_taxontree_views_filters_alter($view, $filters){
 // alter the filter values here
}

So maybe someone needs a similar functionality or maybe this piece of code migth be interesting for integration into the views module as well.

Any feedback very welcome.