As a workaround, you can use the username as a search term. If the person includes their username in their sig, or anyone in a thread addresses them personally, you'll at least get some hits.
[Edit]Or if they edited a post.
Frex, a search on "dumbluck" gives 35 hits.
[ 26 December 2001: Message edited by: capnq ]
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Cap'n Q
"Good morning, Pooh Bear," said Eeyore gloomily. "If it is a good morning," he said. "Which I doubt," said he.