I had no problem with my last trip. I was able to call up the booking with the locator, even though it was made by a TA which services my client. I did need to find the AC record locator rather than the one the TA had, I suspect since theirs was booked through SABRE or some other res system.

However, after I upgraded, the AC agent gave me the AC locator and the itinerary came up, complete with my upgraded segment and seating.

So it will work on reservations not made on the AC web site, or even with AC direct.