The headings will be output as HTML, so you can code a link into a heading text section.
PHP Code:
define ('CAT_BOX_HEADING_23','1|0|||<a href="your desired address1">Manufacturer 1</a>|2|<a href="your desired address2">Manufacturer 2</a>|2|<a href="your desired address3">Manufacturer 3</a>|2'); 
All of these links will appear above category 23, and can be styled using .catBoxHeading2 {} and .catBoxHeading2 a {}.
For internal links, use href="index.php?main_page=whatever&manufacturerId=whatever", or however the URL looks when you are on the desired page.