Thought this might be useful to some on here embarking on European road trips. I was just informed that car registration for non-Belgian vehicles is now mandatory when entering Brussels (diesel engines immediately, I believe this came in during October, petrol engines as from 1st Jan. 2019). You'd need to register here: https://www.lez.brussels/en/content/registration in advance Failure to comply with the entry criteria for the Brussels low emission zone will lead to a fine of €350.
Thanks for that, it looks like the registration is free and lasts for 3 years if you don't change your car. It works on number plate recognition so no stickers are required for the windscreen