Yes, they exist. Dimension produced one for Santana's early Formula disc brake installations which is available from most tandem dealers.
However, the Dimension model isn't always a precise fit, depending on which type of hub you are using and the rear wheel spacing on your tandem. So, what you'd want to do is to send a note to Mark Johnson at Precision Tandems and tell him what year / model Burley tandem and what brand/model of hub you are using. He stocks a wide variety of the various adapters and will be able to recommend the right combination of rotor and caliper adapter.
His Email is: firstname.lastname@example.org
As for using a disc as a drag brake, if you need or want a true drag brake go with the Arai drum. If you are looking to have a disc brake "just in case" for the occasional steep descent, for its wet weather performance, etc... and are a lightweight team the Avid BB7 would probably work out just fine. However, in this application the disc is usually set up as the primary rear brake instead of as a supplemental brake controlled by an extra brake lever (usually a thumb shifter controlled by the captain or stoker).
Again, I'd recommend a phone call to Mark who will be able to talk about your needs and the hardware needed to make your mod, to include some special mods that will improve the performance of the Avid mech. disc brake if you decide to go that route.