I don't know. Usually the box only allows to work with hostnames and other "most easy" options. The user interface is strongly intended for "average people". Its very limited. Maybe there's a chance to 'Blacklist' everything (in parental control). This mode has an additional exception table for collected (blocked) IPs.
Regarding the fake gateway. It doesn't work. The gateway has to be in a known subnet as well. The weird thing is (and that's what i had in mind all the time) that the camera does allow everything in the gateway field and it worked that way before. But that's just a web interface. Who knows what's happening there (ARP cache fallback, guessing...).
Back to square one...