I'm almost 100% sure you - I use zsh, and I know you can do it there. I think there's nothing here that's zsh specific though:

function hosts {set -A reply $($HOME/bin/gethosts)}
compctl -K hosts -x 'c[-1,-l]' -u  -- ping ssh telnet traceroute
[download]

This makes tab completion call my gethosts script whenever I try to tab as the first argument of ping, ssh, telent or traceroute. My gethosts script is pretty simple:

#!/usr/bin/perl -w

use strict;

my $HOME= $ENV{HOME};

open (HOSTS, "$HOME/.ssh/known_hosts") or exit;
my @hosts = <HOSTS>;
close HOSTS;

foreach my $line (@hosts) {
  chomp($line);
  my ($name) = split /\s+/, $line, 2;
  my @names = split /,/, $name;
  foreach (@names) {
    print "$_ ";
  }
}
[download]

So it returns an bash array (a list of space separated strings) of all the hostnames and ips in my .know_hosts file.