Yes! With IPPORTFW, you can allow ALL or only a select few Internet hosts to contact ANY of your internal MASQed computers. This topic is completely covered in Section 6.7 in this HOWTO.
Now that I have IP Masquerading up, I'm getting all sorts of weird notices and errors in the SYSLOG log files. How do I read the IPFWADM/IPCHAINS firewall errors?