Thank you.
Apparently it was as simple as updating to FM 12.0.2.
From the Adobe FrameMaker 12.0.2 ReadMe file:
Feature enhancements
- Generating a list or an index of references (imported graphics) results in a blank file. (Ref#: 3725424)
I don't know that it is accurately called an "enhancement" but more of a "fix" since it has always worked in earlier versions.
Anyway, thanks again!