Eureka! I found the exact answer I needed in this same forum:
www.limesurvey.org/forum/can-i-do-this-w...ale-2-in-dual-matrix
Sorry about not having looked hard enough for it before posting...
I guess the same script that's provided in the thread above can also be modified, so instead of filtering the second scale based on the first-scale answers, other logic could be embedded in the second scale based on the current row in the array.
Even more wonderful would be:
1. For EM to have a variable similar to "this" or "self" that referenced the answers in the current array row. That would solve a lot of logic coding issues around answer options.
2. For answer options to support relevance expressions directly, the way it's done for subquestions.
Cheers to all!