Just be aware that sort would be performed on client-side only. Which means that if you do not display all facet items, the sort will be partially incorrect.
You can use WPSOLR’s facet data event WPSOLR_Events::WPSOLR_FILTER_UPDATE_FACETS_DATA
for that.
You can find an example in the Yoast add-on file, where permalinks are added to each facet url: /htdocs/wp-content/plugins/wpsolr-pro/wpsolr/pro/extensions/seo/class-wpsolr-option-seo.php.
/**
* Modify the facets data at your convenience just before calling the Twig template for display
* @param array $facets_data
* @return array
*/
public function update_facets_data_skeleton( $facets_data ) {
// Do something here on facets data
$facets_data[...] = ...;
return $facets_data;
}