Index: wp-admin/includes/class-wp-plugin-install-list-table.php
===================================================================
--- wp-admin/includes/class-wp-plugin-install-list-table.php	(revision 24117)
+++ wp-admin/includes/class-wp-plugin-install-list-table.php	(working copy)
@@ -84,7 +84,7 @@
 				break;
 
 			default:
-				$args = false;
+				$args = apply_filters('install_plugins_custom_tab', false, $tab);
 		}
 
 		if ( !$args )
Index: wp-admin/includes/class-wp-theme-install-list-table.php
===================================================================
--- wp-admin/includes/class-wp-theme-install-list-table.php	(revision 24117)
+++ wp-admin/includes/class-wp-theme-install-list-table.php	(working copy)
@@ -89,7 +89,7 @@
 				break;
 
 			default:
-				$args = false;
+				$args = apply_filters('install_themes_custom_tab', false, $tab);
 		}
 
 		if ( ! $args )