Canada can’t set up passengers preclearance at US airports unless the US airport is commercially for it or pushed to go for it by nook or by crook.

The terms of US-Canada agreements relevant to CBP Preclearance in Canada are not completely bi-directionally equal.

CBP officers assigned abroad do get some special treatment. I’ve seen a bunch of US diplomatic passports for CBP employees working outside of the US, including assignments at foreign airports with CBP Preclearance or IAP type work.

If Belgium decides to go for this, I hope Belgium pushes for and gets a complete and comprehensive pre-waiver from the US when it comes to criminal and civil immunity protections for CBP and other DHS employees working BRU and for their relatives regardless of assignment/employment relationships of those on or accompanying a foreign tour of duty.