As a follow up, I just finished a long layover in Brussels on my way to Porto. We arrived at 7:30am from Dulles. It took less than 5 minutes to clear immigration.
When filling out the PLF form, I accidentally said that I was not on a connecting flight. This caused the airline to require my PCR test at check in at Dulles. My wife did not make this mistake and she was not asked for her PCR test.
On arrival in Brussels, I was asked for my passport and the QR code of my PLF. The barcode scanner did not seem to be working so they were waiving everyone through as long as they could show a PLF QR code on their phones or printout. We exited the airport and enjoyed Brussels for a few hours.
One note. There was no luggage storage at the airport. We took our hand carry luggage with us to Central Station thinking we could check them there. Unfortunately that luggage storage was also closed. I was told only Midi Station has luggage storage. We ended up using Stasher instead.
YMMV.