Code:
ON &^*:NOTICE:*:*:{
  if (!$window(@notices)) window -lez @notices
  aline -hp $color(notice) @notices $asctime([HH:nn:ss]) $nick -> $+($target,@,$network,:) $1-
  editbox @notices /notice $nick :
  haltdef
}

menu @notices {
  dclick:editbox @notices /notice $gettok($sline($active,1),2,32) :
}


This fills "/notice nick :" into the notices window. (For the 1st time you'll need to close @notices so it can re-open as a -l list window.) Note that it also fills the editbox when you double-click on that line in @notices. You may want to remove the line filling the editbox as each notice is received, since that creates the risk that person2 sends a notice while you're composing notice to person#1 and wiping out your text. At least it won't send the message to the wrong person instead of zapping it.

/onotice is simply a built-in short-cut by mirc to send a normal notice to all ops in the active channel. Since @notices isn't a channel, mirc has no way to know what channel you're referring to.

what you're doing is accidentally discovering a 2nd way to send notices to a group of people, and the receiver can tell the difference.

When you do "/onotice message", receivers cannot tell the difference compared to your doing a separate "/notice nick message" to each recipient, and they see $target as $me.

When you do /notice @#channelname message", the receiver sees target not as $me but as @#channelname. Different ircd's in the past have supported or not-supported different variations of targets, and they can neglect to support a target without warning and instead blab the notice to 100% of the channel. You should also be able to do @+#channelname to send the message to all ops+voices at the same time, and might also be able to send message to superop/protected ops with the & prefix with /notice &#channelname. You might have to use a clone to see which targets are received by the intended prefixes, and whether it also bleeds to be seen by 100% of the channel.

The code can be changed to fill the command as the sending target which could be @#channel but your mirc can't tell whether someone uses /onotice or send a specific notice to only you.