@mirus' answer regarding the shortcode gave me the idea to check out how WooCommerce is querying only the on-sale items. Apparently WooCommerce has a wc_get_product_ids_on_sale()
function that will return the IDs of the on-sale items. Then we can easily adjust the query using the post__in
parameter to only return those specific items.
WooCommerce has a woocommerce_product_query
hook in the class-wc-query.php
class that allows for us to modify the query before it is run.... it is run on pre_get_posts
which is the usual place for modifying the query. Using Woo's hook just means you let them handle the majority of the conditional logic about when this query modification should be applied.
add_action( 'woocommerce_product_query', 'so_20990199_product_query' );
function so_20990199_product_query( $q ){
$product_ids_on_sale = wc_get_product_ids_on_sale();
$q->set( 'post__in', $product_ids_on_sale );
}