I just learned the hard way that AmEx bookings are considered "non-designated GDS" bookings. In fact, my upcoming one (on a prepaid but non-opaque rate) is actually labeled "Expedia Com Direct Connect" in the Hilton system. When I called, I was told in no uncertain terms that I could not attach my HH number and that I wouldn't earn any points. So I guess I'll grovel for on-property benefits at check in, but keep my expectations low.

I realize that hotels are getting even more aggressive than airlines with the carrots and sticks for direct booking, but am just as surprised as the OP to see AmEx treated as punitively as Expedia given the CC relationship with HH.