What about just finding threads started by a certain member in select forums rather than all forums? For some members, this can make a substantial difference in the search output.
In that case I would go to http://www.elitetrader.com/et/index.php?search/&type=post and type in the username of the whoever you want to search for into the "Posted by Member" box, and then select the forum you want to search in, and then click the box called "Display Results as Threads". Now what you'll end up with is all the threads that user has participated in within that forum, but in the actual search results you'll be able to see which threads that user started and which ones he didn't. It's not exactly the search result that you're asking to achieve, but it's about as close as we can get for now.