Coded a nicer fix
My filter is called filter_regions (that is its ID)
I created a button set like this
<button class="chosen-toggle select" id="globalCheck">Globally Available?</button>
<button class="chosen-toggle deselect">Deselect all</button>
then added this script at the end of the view
<script>
$('.chosen-toggle').click(function(){
$('#filter_regions option').prop('selected', $(this).hasClass('select')).parent().trigger('liszt:updated');
});
</script>
We now have a button that selects all options / and one that clears them
