If this was possible, neither I nor the second agent I spoke to could figure it out. AmEx's rate for standard economy was about 10% lower than the published fare and not significantly higher (<$100pp) than the econ light fare in this case-- as STE+, the fares are indistinguishable for me when flying AF/KL.

I did look at other routings where the difference would be significant (several hundred dollars per person), disappointing that AmEx's system does not seem to support light fares.