Not to step on any toes, and certainly not to try to hijack the thread, but... Just about any medium and heavy truck manufacturer that deals in Europe will have these parts. They went to air discs as standard instead of drums DECADES ago. If somebody wanted to install an air-powered driveline disc brake, or even convert their entire vehicle over to discs at all wheels, it's not hard to find the parts to do so. Even Meritor right here in the US provides air discs on some axles.
I'm not out to be argumentative here. The reality is that disc brakes are quieter, more reliable, cooler, more efficient, easier to maintain, and less expensive to work on, than their drum equivalents. We've known this for a great many years.
Here is a very good article on the subject:
http://www.todaystrucking.com/whats-stopping-you-the-4-1-1-on-air-disc-brakes
If a person wanted to put air discs on his driveline, it's definitely within the realm of possibilities. And would certainly work better than the other suggestions that have been made.