[BirdOPrey5]

There is no userid stored in $lastpostinfo...

Here is a copy of all the info in $lastpostinfo-

[forumid] => 9
    [lastpost] => 1293022161
    [lastposter] => BirdOPrey5
    [lastthread] => my test thread
    [lastthreadid] => 56
    [lasticonid] => 0
    [threadcount] => 2
    [replycount] => 3
    [lastpostid] => 201
    [lastprefixid] => 
    [forumread] => 
    [styleid] => 0
    [title] => Test Forum Y
    [title_clean] => Test Forum Y
    [description] => This is a test forum with a description.
    [description_clean] => This is a test forum with a description.
    [options] => 89799
    [showprivate] => 0
    [displayorder] => 23
    [daysprune] => -1
    [newpostemail] => 
    [newthreademail] => 
    [parentid] => 8
    [parentlist] => 9,8,1,-1
    [password] => 
    [link] => 
    [childlist] => 9,-1
    [defaultsortfield] => lastpost
    [defaultsortorder] => desc
    [imageprefix] => 
    [excludable] => 1
    [podcast] => 0
    [depth] => 2
    [permissions]=> Array

You would need to make some plugin on a forumhome hook to find the userid given the username which is available in: $lastpostinfo[lastposter]

I did some work on this a while back and never fully finished it but I think what I have works if you want to take a look- (attached) - feel free to use it however you want.