Looked at the Bicycle Designer YouTube video of the product: http://www.youtube.com/watch?v=ZFl5o6OyUTw
If you advance to 7:30, you will see the parts that they used for their kit.
Formula makes a 10" (255mm brake disc rotor). Santana Tandems installs them on their bikes: santanatandem.com/Library/SelfAdjusting.pdf
Their Harmony adapter looks like King's Sales & Service TopHat adapter. Their Secret Adapter looks like the brake disc adapter from Atomic Zombie with threads cut in the inside to adapt to a freewheel hub.
I have no problem with the prices. Add up the cost of the individual components, time, labor and design and they're not making a boat load of money. $288 per unit isn't too bad. My guess is that they had the harmony adapter and Secret Adapter custom machined.
The only issue I have is using a flip-flop hub which has a right hand thread on the rear hub. I would prefer a left hand thread. I wouldn't want the whole shebang coming apart during a high speed stop.
From reading on the Internet, you'd want to use a dry chain lube on your chain, so you don't contaminate the disc brakes.