I have discovered you can do redirection in m0n0wall using a workaround:
Create a regular inbound NAT like for the redirect you need and leave the
interface as WAN. Comment this with something unique, so you can find it
with ease in the config file.
Then make a backup of the configuration, save the file and then edit it.
Look for the unique comment and you will find the new inbound NAT entry. To
change it into a redirect, just replace "wan" in the interface field into
"lan" (without quotes). Make sure it is the correct redirect you need and
not one of your regular inbound NAT entries.
Save the file, restore it into m0n0wall and after reboot you will have the
I have used this with no problems, but not with squid.
If you need to remove the redirect, remove it from the NAT page, where you
will see it has no interface listed. I have also found out it is good
practice to reboot the m0n0wall after removing a redirect.