Proto source port destination port gateway Schedule Description
* * * * * * DMZ
So setting the above for an interface ('Choose on which interface packets must come in to match this rule.') will mean that interface is no longer firewalled.
Thanks :)
PS. It's also worth mentioning, in case anyone uses this, that it would be important to stop the above interface 'talking' to the LAN like this:
Proto source port destination port gateway Schedule Description
* DMZ net * ! LAN net * * Permit DMZ to any BUT LAN