The only source of information available to Lua plugins for whether your friends are logged in is the chat window.
So, when you first log in, there is no way for a plugin to know which of your friends are online. It also can't know which people are on your friends list; you would have to manually enter them.
However, a plugin could easily monitor the chat window for messages such as "Your friend X has just logged in" and display a big message in the middle of your screen (I think there is already at least one plugin that will do this), or update some sort of window as described in the OP.
At login, there is a clunky way that a plugin could find out which of your friends is online. Each time you log in, you would have to click a button n
times, where n
is the number of friends you have. This button would execute the "/inspect X" chat window command to find out whether each person is logged in.
Edit: Check out Garan's LotRO Alerts
plugin, which allows you to monitor the chat window for specific messages, and blow them up in the center of the screen.