I'm really not sure of this. There isn't a specific USB and workgroup name that is configurable in this type of application. There is SharePort and Cloud access when thats configured. All I know is that if a USB drive is connected, on a MAC or PC, we can map to the drive by using SMB://192.168.0.1/A1_USB or in Windows: \\192.168.0.1\A1_USB. It would seem that interaction of the NAS code and programming of how it behaves and discovers USB drives and possible and if the 868L automatically sends out information when a USB drive is connected could be a point of contention and possible that it might not work. Only way would be to test it out and see. I presume that if you connected a USB drive to a online PC as a test, the NAS would discover the USB drive correctly?
I personally don't use the USB connections for any back ups or large file xfers as USB is not the best in performance compared to wired LAN networking. If your not too concerned about performance then don't worry about it.
I really can't offer much more advise at this point. I would highly recommend that you phone contact your regional D-Link support office and ask for level 2 or higher support and see if they can offer you more information and help regarding this if your not going to buy and try one on for size. You might also get a hold of of the Mfr of the NAS and see what they have tested, recommend and support. I presume that they might have done testing on some level with all the various Mfr routers with there NAS, maybe.

Keep us posted.