Amex Travel is operated by Expedia. Does Expedia have the Light fares?
That's a great question. I was not able to pull up any between my city pairs, but other routes have the economy light / main cabin distinction. Those do not seem available via AmEx travel-- currently I checked KBP<->ORD Dec 3rd to March 13th.
On that route, Econ light shows up on Expedia and is about $185/pp less than the AmEx fare-- the AmEx fare is about $15 more than Expedia FWIW. Maybe an AmEx agent could access the light fare on that route-- unknown.