Just did the run PIT-ATL-NRT-HKG-DTW-PIT for the 2x promo, and as Platinum I could not get bulkhead prior to day of departure. I was lucky and got it for both the long hauls on the triple-7 (30-J going there and 30-A coming back), but I couldn't get them assigned until the Gate Agent had control of the flight .... couldn't even get them at check-in and couldn't check-in online because the flights were international.

Now, since we all know that Diamond's can walk on water, they may be able to get a bulkhead pre-assigned. The other option may be somebody who has a very large corporate contract can make the right call to get the seats assigned - but other than that you'll have to try at the gate. Only recommendation, if at all possible, is to be 1st in line at the gate to get the seats. Keep in mind the GA has the power to re-assign passengers, but if you've got a High Value Customer there they may not move them. YMMV