TandemGeek

Yes, but you still need to address everything else first given that it's your front brake that's the true source of your problem.

A rear brake, regardless of what type, will not STOP a tandem. So, as to whether you'd be better served by a disc or drum for your future trip the question is, what's the combined weight of you and your wife (value #1) and what is the most weight that you will carry on the tandem in terms of loaded panniers (Value #2) or in a trailer (Value #3)?

Unless your tandem will be carrying upwards of 350 - 400 lbs into steep terrain OR if you and your wife have decided that all steep descents need to be taken at very low speeds that require constant braking, a drum brake is typically unnecessary. I say typically only because there is always an exception to prove the rule. A drum brake can sometimes be found on places like Ebay second hand for between $50 and $150, depending on the condition and if any parts are missing. An example of pricing for new ones can be found HERE and some additional general information can be found HERE. Your tandem is somewhat ready to go for an Arai in that it has a left-hand threaded Shimano HF07 or HF08 tandem hub, a pac-man braze on for the reaction arm and a bar-end shifter which is what's typically used to control drum brakes.

As for the disc, a rear disc brake is typically used to replace the rim brake not to supplement it. The disc will be more effective in wet conditions at slowing your tandem, however it still can't safely stop a tandem by itself and can also cause a rear-tire skid on wet pavement, particularly in corners; again, the front brake is your most important brake. The disc will also tolerate a significant amount of brake heat if used for prolonged periods of time on a steep decent; but, again... it's not designed to be used as a drag brake. Cost wise, probably about $145 for the caliper, a 203mm rotor, a thread-on rotor adapter and perhaps an inline Travel Agent to make adjusting and using your Avid a bit more easy.