[kh99]

Well, you want to look at includes/class_bbcode.php. One thing you might do is look at using using hook location bbcode_fetch_tags to modify the data for each tag, maybe substitute your own function to handle a bbcode. Otherwise you could use hook bbcode_parse_complete but then I guess you're just working on the entire post. Of course you can always edit the file if there aren't hooks in the palces you need.

The function that's called to do the work is parse(), but it might save a bit of time to look at build_parse_array() and parse_array() (if you follow it though you'll eventually end up there).