GamblerGORD53, I'll take that bet. This coming summer I will be loaded going through the UP of Michigan. The hills are short, but steep and I will have ample opportunity to test it in a non-mountain region. Speeds on those hills with an unladen bike can reach 50mph, albeit for a short distance.
The question is, when the drag brake proves successful, what is the payout?
Note, when we sold tandems we stocked Santana. They had a 200+mm rear rotor to be used as a drag brake, and the other two brakes were rim brakes intended to stop the bike.
Drag brakes are not a regular "third" brake as they are not in service to be used to stop the bike. They are only there to help control speed, and are very effective at doing the job.