Yeah, those functions can’t be simply overridden (like the functions in wp-includes/pluggable.php).
Your best bet is to do the following:
1) Copy the entire function you want to replace yo your functions.php, and change the function name (to something like custom_directory_members_search_form() )
2) Make sure that the template file that calls the template tag (I think it’s members-loop.php, though I’m not in front of my computer at the moment) exists in your child theme. Replace the reference to the original function in that template file to your newly created function name. Then you can customize to your heart’s content.
