If I'm understanding you correctly you want to be able to send messages to someone while they're offline and have them relayed to that person when they next come online. If that's the case I'm afraid that's far beyond mIRC's control - it would require some big changes to the IRC protocol itself.
One could check if the network already supports this:
/memoserv help
[07/04/05][13:06:43] -MemoServ- ***** MemoServ Help *****
[07/04/05][13:06:43] -MemoServ- MemoServ allows users registered with NickServ to send
[07/04/05][13:06:43] -MemoServ- each other messages, which can be read by the recipient at
[07/04/05][13:06:43] -MemoServ- his/her leisure. Memos can be sent to people even when they
[07/04/05][13:06:43] -MemoServ- are not on IRC.