I think I was able to perform local loopback with the old Westell WireSpeed B90-36R516 modem, but not with the replacement Siemens SpeedStream 4100.
However, SBC used to allow two simultaneous connections; one DSL and one dial-up. I would take my old SMC Barricade 7004BR, which has both parallel and serial ports, disable the DHCP server, and connect one of the LAN ports to my regular router. Then I would connect an old SupraFAXModem 288 to the serial port of the Barricade.
From the Barricade GUI, I'd issue standard Hayes modem commands to the SupraFAX, and establish a dial-up connection. Then I would look at the Barricade status page for the dial IP address. With that I would execute a 'route add' command at a command prompt, assigning the dial IP address as the gateway to my DSL public IP address block. This last was important, because it allowed me to loop back to my DSL IP address through the Barricade.
Assuming AT&T still allows it, one can use a computer with both an Ethernet adapter, and a dial-up modem. You have to break the U-verse connection first; disconnect, and don't allow a reconnect. Then dial a connection with the modem. Finally, with a functional dial connection, re-establish the U-verse connection. This will ensure that the default route gateway for all traffic to the Internet is through the U-verse device. Windows uses the last Internet connection made to build the routing table.
After that, you determine the U-verse IP address, and use it in the 'route add' command. Limit this to just the IP address on the U-verse RG. The modified route will send client commands to the RG through the modem.
Simultaneous dial and DSL connections.
~Oh Lord, why have you come
~To Konnyu, with the Lion and the Drum