THe(sic) Behavior I am looking for would be the Pi looking at the lan0 on startup and trying to pull an IP from DHCP. If successful Use it, if no DHCP then act as a Piratebox. I don't know if this would involve bridging to Wlan0 or hosting its own IP range. ( I am a fan of simple)
Source: https://forum.piratebox.cc/read.php?7,21752,22587#msg-22587