Add an action at the end of WP_Query::parse_tax_query() called (wait for it...) parse_tax_query which will enable developers to alter $this->tax_query->queries before SQL is generated. pre_get_posts is too early for many scenarios.

git-svn-id: https://develop.svn.wordpress.org/trunk@25311 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/query.php

@@ -1887,6 +1887,8 @@ class WP_Query {
 		}
 
 		$this->tax_query = new WP_Tax_Query( $tax_query );
+
+		do_action( 'parse_tax_query', $this );
 	}
 
 	/**