Again using DDE, this time the sender is the mIRC in the channel you want to monitor.
As in Onlines example, using dde names "mIRC1" (receiving) and "mIRC2" (sending)
on *:join:#channel:{
dde mIRC1 command "" /announcejoin %announce1 %announc2 %announc3
}
In the receiving client
alias announcejoin {
echo -a * mIRC2 sent: $1-
}