ok, you can use the wl-hdd as 'router' i'm not sure which things are disabled or enabled by default and at the moment their are no custom firmwares yet
if you want something to have tested ask me and i'll try to test it for you
i've tested this setup a week ago:
Internetline connected to WL500g in hybrid WDS mode
WL-HDD connected via Hybrid WDS(i disabled DHCP at wl-hdd) to WL500g (no wires!)
Asus Notebook connected through wire to the WL-HDD and i had access to the network/internet