CSS Horizontal Dropdown Menu With jquery

I absoltely love this contrib and am currently trying to get it setup and configured to my liking. It is all fine, and I am opting to use the alternate_ui_generator in order that the dropdown menu will show my categories... However I do not want it to show all my categories from the root category (0) down... I would like to specify a pseudo root category to treat as the top level and thus then have it only traverse categories below that specificed top level. I.e. I have 10 top level categories, but I don't want them all to be on the nav menu... I would like the nav menu to treat top level category 6 as root, and thus look at all the categories below it for the purpose of constructing the categories accross the nav menu. Obviously all sub categories would still come through in the correct way. It is just that I want to force a certain top level category to be the starting point. I can not easily work out what PHP code I need to write in categories_ul_generator.php to do this... I need some sort of more clever loop logic but need some help/advice please.

I am using Zen 1.3.9f.