You could write a program which crawls through the whole forum by doing these steps:
Code
function crawlIt($url){
-> parse the source code
* show all images (<img> tags)
* save all urls in <a>-tags into an array
for each element of the url-array (maybe only <a> tags, that will lead to sub-sites of the board) -> call crawlIt($url_of_<a>_tag)
}
Could take a huge amount of time, depending on how big the forum is. Also your IP could be banned due to flooding the server.
i think there is no other way of doing this than writing a program.
maybe you can check the user's personality, hobbies.. etc. if he/she's an admin they might have hidden it in a rules thread. check in which sub-forums they are active and search for it there.
check their latest topics/posts. maye if you are a programmer and you use my method you can check the crawled pages for a string like 'last edited by user at date-xx-xx' which would make it a lot easier.
apart from that -> too little information