This won't work on some servers!
mIRC's onotice command sends a mass notice and NOT notice @#channel!

Something like this will work everywhere ( I think and I hope... ):
Code:
alias vnotice {
  var %i = 1 , %nicks , %n = $nick($chan,1,v) , %maxtargets = 15
  while ( %n ) { 
    if ( %i % %maxtargets == 0 ) { 
      notice %nicks $1-
      %nicks = %n
    }
    else {
      %nicks = %nicks $+ , $+ %n
    }
    inc %i 
    %n = $nick($chan,%i,v)
  } 
  notice %nicks $1-
}


Usage: /vnotice <text>

%maxtargets set to 15 is the maximum number of users you can mass notice on a server. The server I usually login (CR) allows 17!
The alias sends one command per %maxtargets voiced users. Too many users may flood you out of the server!

(I don't know if mIRC's onotice checks these things out cause I've never onoticed that many ops with mIRC's command...)

Maybe I wake up one day to notice that all my life was just a dream!