To tie off this thread for good, I thought I'd post the outcome of our search (better late than never, right?).

We found a shop in Weingarten, Germany (http://weingarten.feineraeder.de/). The proprietor was incredibly helpful in helping us out, and even went so far as to pick us up at the train station and put us up for the night in a spare bedroom (I suppose we sounded desperate!). In the end, we purchased a slightly used Zwei-plus-Zwei touring tandem, which saw us through the remaining 4000 km of our 2004 trip, and a number of other trips since.

Thanks again Brian and James for all of the helpful directions and suggestions!

Brad