Looks like R065 has improved this slightly, at least visually, as I was able to enter 4 port mappings before the GUI started going to a a white screen every time I pressed Save to record the 5th mapping (in both the simple GUI and the OpenWRT screens). Not that I was able to test the functionality of first 4 maps (ran into what is hopefully a DHCP configuration issue but won't be able to test further until tomorrow evening)