Ambassador, British Airways; FlyerTalk Posting Legend
Join Date: Apr 2012
Location: Leeds, UK
Programs: BA GGL/CCR, GfL, HH Diamond
Posts: 42,964
If it is a booking made using an FTV you should be able to make seat reservations whilst waiting for re-ticketing if you are a OWS/BA silver or higher.
One thing I have noticed, sometimes my BAEC number has fallen out of the booking between when I call to make the booking and the ticketing and this would prevent me doing seat selection, but you can put it back in on MMB and then do seat selection.