greetings,
I don't know what RedHat exactly does to initiate the firewall, and moreover _what_ firewall it is using. But in case it's iptables then you could "search" along the bootup sequence and find out what firewall script is used to fire up iptables. Start to look for files in the "rcX.d"-directories (whatever your regular runlevel is, normallly "3"->"rc3.d" and check for files like "S99iptables" or whatever; that's under MDK here). Doing so you will find out which file is passed to iptables, containing the firewall rules..
Once you've found it, it should be easy to add 2 apropriate rules (outgoing, incoming) for uncommon ports. If the script's somewhat reasonable it should have the port-familes specified in groups anyway (like $UNPRIVELEGDED=1023:65535, $SSH_PORT=22 or something like that).
In case there is no rule set for NFS (in case NFS is handeled by another "top-level"-gouverning rule) you can still launch some tool like "Firewall Builder" or the aforementioned "Firestarter" (not tested this one though) and examine the structure of their ruleset.
hope this helps