There's probably pre-existing scripts out there that do (most of) what you're looking for - google for 'mirc memo'.
E.g.
http://www.hawkee.com/snippet/2779/The basic components you'd need are...
- An alias that actually adds notifications
- An alias that checks for notifications for a person once they join the channel
- A remote 'ON TEXT' event to handle users wanting to send a memo
- Entries in your Popups script file to handle your channel right-click. You can use the $?="" method to get popups that ask you for the nick/reason.
