The drum brakes on tandems are generally not used to stop the bike, but rather are used as "drag" brakes on long descents. Instead of putting all that heat into the rim and blowing a tire or something, the drum is set (usually with a barend or thumb shifter so you don't have to hold a third handle) to control speed. No where near the stopping power of your cantis. Its usually a third brake and (I believe) never a primary.

No experience with "internal" drum brakes, but I imagine its much easier to engineer them t be worse than what you currently have as opposed to better (wow, did that make sense???).

Have you considered a decent linear pull or V-brake depending on the brake levers (blind, can't see images)? Used hydraulic Magura? wiggle.com lists clamp-on disc brake conversions to put caliper mounts where there were none...If you were going to replace the front hub you might be able to go disc with your fork, but I've never used these, don't know if I would trust them, and may catch flack for mentioning them. What about both the cantis and the drum brake hub with a "Y" connector?