Hi Marcus, and welcome to the Thorn Cycling Forum!
I'm in a similar boat with my 1989 tandem and Arai drum brake. I know eventually the Suzue freewheel hub will need replacement and then...? Some of the newer tandem rear hubs with threads on the left side are indeed incredibly expensive.
For awhile, I was in negotiations to have an adapter made to go from a 6-bolt disc hub to a threaded Arai mount. Things looked promising for awhile, but fell through. I'm now looking at the problem anew and wondering if the Arai's backplate could be modified for a 6-bolt disc hub. I've found the Arai to be really helpful on long, steep descents. With its SunTour bar-con actuator at full lock as a drag brake, it keeps the speed down to around 68kph/42mph. Without it, I commonly hit speeds over 101kph/63mph with a stoker aboard on the steep hills surrounding Eugene on three sides. I use SunTour and Scott/Pedersen SE cantilevers along with the Arai. Drums can absorb a *lot* of heat and still work well. I've seen mine glow quietly after long nighttime descents.
Past post on the topic here:
http://www.thorncycles.co.uk/forums/index.php?topic=7115.0Of course, Arai is no longer. An (expensive) replacement is now available in the Maddock Machine "Mad Dog" drag brake, manufactured just up the river from my home:
http://tandemgeek.wordpress.com/2010/11/14/next-generation-drag-brake/http://tandemgeek.wordpress.com/2013/07/10/next-generation-drag-brake-now-available/http://www.tandemseast.com/parts/brakes.html#Maddox drum
Back to hubs: Here's a list of cassette hubs that are currently available *and* Arai-compatible:
• Phil Wood (40, 48 drillings)
• Shimano (HF-07 and -08 in 145 and 160 OLN, 40, 48 drillings)
• White Insustries (Daisy and Jockey models, 40, 48 drillings)
• Hadley (40, 48 drillings)
• Aerospoke 26in tandem wheels can be had in a model that is Arai-compatible
• Chris King (maybe, made in Portland just north of me)
• DT Hugi (maybe)
What I'll probably do with the Suzue is use it till the ball races are gone, then knock out the ball race and mill the opening to take a sealed cartridge bearing and press that into place. Should work so long as my stock of 7-sp SunTour AccuShift freewheels holds out. A lot cheaper than buying one of the options above, though it is really just postponing the inevitable.
Given the costs involved, if I were in your position at present, I'd lay in a stock of spares for the Shimano and figure on it raking me through the Pamir and beyond if serviced periodically. Remember, the right-hand bearings are in the freewheel part of the freehub, and the left side has adjustable cones. Therefore, the thing to ultimately limit you would the be the condition of the left bearing race, and it could be milled and replaced with a sealed cartridge bearing as I plan. I've done it once before on a single-bike hub with good results.
If you do go with a replacement, I think I'd choose the Phil FSA (Field-Serviceable Axle) Arai-compatible tandem hub, and take a spair pair of bearings with me in case field-servicing is needed.
In case you're wondering about the other side of the equation, Arai brakes can be relined at any friction materials shop (automotive clutches, brakes) and I once knew of several moped brake shoes that would fit reasonably well with a bit of edge grinding. be sure to grease the cam actuator with just a dab of high-temperature brake grease that won't run when hot, and replace the friction materials before the wear goes into the shoe itself.
Best,
Dan. (...who would rather his tandem brake than break)