It will randomly change your fullname everytime you connect to server. Just create realname.txt in your mIRC dir and put in the names you want.
on ^*:LOGON:*: .fullname $read("realname.txt")
Actually, On Logon event would be right to use.
From the help file:
on ^:LOGON::echo Logging on to $network $server
Triggers before mIRC sends the standard logon messages to the server. If you /halt this event, mIRC will not send the standard logon messages, allowing you to send your own messages.
On Connect events will only trigger when you just got connected to a server.