If there is someone that joined before me, and I didn't who/whois him/her before executing this alias, I won't get any data from that user; but I think this idea would be especially useful on "on join" events.
As you've said, if it could be only done by who/whois, I think it wouldn't make any sense.. it's a pity, this 'identifier' could make many scripters life a bit easier
