Just to share my experience at MIA. Arrived with BA and took the train to the immigration at Concourse D. After visiting the officer at the booth I informed him that I wanted to complete my GE.
I was given an orange paper (normally this is for those with food items or above personal allowance for entry) and directed to a special lane (for these additional check).
At the bottom of this was another officer who will check the reason you are in this special area - it is behind the baggage belt (as you can see the usual folks on the other side waiting for their bag).
After collecting your check in luggage, following the path, will bring you to the additional check/security area - you will be called and wait.

There did not seem to be any priority or separate desk for GE applicants. Seems like you will be processed as they determine.
In my case, there was another pax on my flight who did not seem to have check-in luggage and had the same situation. After 5 mins of arriving into the office, she seemed to have completed her interview about 5 mins after that and came out from some "interview" rooms.
After her, I waited for maybe another 20 mins before I was called by the same officer who conducted the interview with her.

I was brought into the same interview area and the officer processing my case was actually surprised as this was his first time getting applicants from the Netherlands and did not even know that it was part of the program. Anyways, after some rudimentary questions, I was out within 10 mins.
I tried to apply it to my connecting flight, but sadly the system did seem to have been updated.
I received the confirmation email about 3 hours later. From than on adding the KID into all my upcoming reservations had no problem getting me PreTSA.

All in all a good experience. Hope this will be useful info for those travelling via MIA.

Cheers!