This is the defacto mass highlight I use to annoy the fuck out of people when on IRC with irssi. It’s good stuff. Modify it to say something incredibly offensive and use it yourself.

use strict;
use Irssi;

sub cmd_hilight {
        my ($data, $server, $win) = @_;
        my $channel = Irssi::active_win()->get_active_name();
        my $foo = "\cBHey,\cB ";
        foreach ($win->nicks())
        {      
                next if ($_->{'nick'} eq $server->{nick});
                $foo .= "\cB\c_$_->{'nick'}\cB\c_ ";
        }
        $server->command("MSG $channel $foo");
}

Irssi::command_bind('mh', 'cmd_hilight');