[Plugin: Query Multiple Taxonomies] multiple select in dropdown boxes

First of all: Great plugin, thanks scribu.
I would love to be able to select multiple terms per taxonomy when shown as dropdownbox.
I tried to extend widget.php by simply changing
.html( 'select', array( 'name' => qmt_get_query_var( $taxonomy ) ),
to
.html( 'select multiple=multiple', array( 'name' => qmt_get_query_var( $taxonomy ) ),
which worked out in the view (using the widget inline on a page)
but gives wrong results:
Specifically the plugin just takes the last one of the multiple terms selected into account for the query.
How can I get the query to consider the additional selected items per taxonomy?
Thanks so much for the help.
Will be willing to donate for that extension.