Im not too sure if this is possible with the osu! IRC-server (it really should be) but generally speaking I would think that you could just put all your friends in a list and once you join the server you send a /whois request for each of your friends to the server.
If someone is offline it will just reply with "No such nick" and if they're online it will tell you in which channels they are and what their idle time is.
This obviously doesn't tell you when someone comes online but it can easily give you an overview of who is online.
You can also try using /names (probably too dumb to filter) or /ison <nicknamelist> (that's probably the best in case it works)
Like really, just dive into IRC-commands and client-utilities a bit, there should be some options that work without peppy having to do anything.