Agent is definitely wrong. I had an award flight on hold for most of this week, in J. Pulled up the booking using the extended search in MMB, and went to try and allocate seating - only to get a "We regret you can't do this at this time, the best choice of seats will be available 24 hours before your trip" message

Took a quick look, and realised that my BAEC number hadn't been entered. Popped that in, clicked the seating button again, and duly was able to allocate myself seats as normal. ^

So the system definitely doesn't allow pre-allocation for vanilla award bookings - only those which have a suitably shiny bit of BAEC/oneworld plastic in them....