If you are looking for markup smileys then you just need to add them into the icons directory. Then you need to edit ubbt.inc.php. The do_markup and undo_markup functions is where the smileys are converted so you just need to add the new regular expressions in there. Just look for the following in both functions
// Convert the smileys
The hack you are working on sounds great. There are still only a couple of search engines that can search dynamic pages so this would probably come in very handy.
