If you chat script is called "chat.php" then you need to add this line into the online.php language file:
$ubbt_lang['chat'] = "In the Chat Room";
What I did to eliminate them dropping off the list after 10 minutes was make the chat page open a pop under... the pop under refreshes every 9 minutes and updates the Who's online info.
I think Dalantech included my popup in his chatroom mod posted here.
