Tandems Forum
OP,
You might have better luck asking on and searching the Tandem forum, and perhaps a Tandem-specific shop.
My Trek T1000 has a disc-mount adapter support as well as the pacman tab for drum brake. I started down the path of adding a disc brake rear, intended as a drag break and not to replace rim brakes, which IIUC your later post made clear is what you're hoping to do.
I should tell you that multiple experienced tandem posters chimed in to say that a disc, even large, was NOT a drag brake substitute for descending on a loaded tandem and avoiding brake fade. Some teams reported no problem, if course, but hills and weights and speeds are not likely to be the same. And it's certainly a separate place to be scrubbing heat, so would have to be better than rim-only.... Lots of uncertainty that probably reinforces why a shop was hesitant.
Forum consensus seemed to be that the T1000's disc support existed for those wanting to swap out rim brakes for discs. I never found a Trek-source discussing it.
There are/were threaded adapters to bolt a disc-mount to the hub. I'd be hesitant to do that if the frame wasn't built for that load on the frame member.