output_add_rewrite_var_callback?

Hi,
Just wondered if a callback variant of output_add_rewrite_var() has ever been considered for inclusion in PHP.

The callback prototype being of the form

rewrite_callback($href, $name) { return 'value'; }

output_add_rewrite_var_callback('foo', 'rewrite_callback');

possibly with the $href as a reference to allow complete replacement.

Jared