Try
Guild Greet. Even if you don't use the greet function it can read the notes to list alts/mains (or you can manually assign them in Guild Greet).
This will only show them when they log in or out.
Prat has a function for this as well, but not really worth it to get it just for that feature as it's a whole compilation of chat tools.
Might be other chat tools that are smaller that include that as well. Hope this helps