I think it's easier than you think. If you have a hotspot LAN network, you can simply connect an access point to this network without configuring any IP address just make sure that the access point is in AP (access point) mode. That's it. IP address is not needed because there is no routing, the access point will simply connect your hotspot user to your hotspot lan network. If you set an IP address because you wish to reach the APs admin page make sure you use IP addresses outside of the hotspot network (you can use for eg.: 10.0.0.1, 2 etc).
The dual ssid (hotspot+internal) can be simply solved with dd-wrt. I don’t think IPv6 will be supported in hotspots soon because 256*256=655536 (192.168.x.x) will be more than enough IP addresses for hotspot users..
