So you mean you've got DD-WRT working in client mode.   I do exactly that with the U-verse RG and I have absolutely no trouble with U-verse identifying devices behind my DD-WRT bridge individually with their separate MAC addresses.   The devices show that they're wirelessly connected. 

On the Wireless tab, I have:

"Wireless mode" set to "Client Bridge"

"Wireless network mode" set to "Mixed"

SSID set to my SSID

Network Configuration set to bridged.

I have v24rc4