My experience is that it is possible to connect multiple access points to a DD-WRT router with the [dd-wrt.v24-22118_NEWD-2_K2.6_mega-nv64k.bin] firwmare.
To enable the HotSpotSystem login on these APs, you also need to set LAN (=> cable) as the DHCP interface in Services->Hotspot->HotSpotSystem. This worked for me on a Linksys E900.
