JONES Etienne H.L.F.
Everything Sylvia has said is right. My own experience has been that sometimes something is possible through personal contact with local officials in the Archives, especially if you are from the same family, even not directly at all, or with a not-official power of attorney for another person who can visit the Archives. But it does not always work, it depends on the municipalities.
BRUSSELS is made up of BRUSSELS Center and 18 other municipalities, my past experience is limited to BRUSSELS Center, until the 1947 census. In some other municipalities everything has been denied to me, and yet searched people were from my family.
Unconventional methods must then be used, for example the phone directory. I thus found 2 JAKUBOWICZ (only 2 probably because the directory doesn't necessarily mention everyone, especially people with only a mobile and no more landline) :
Charles JAKUBOWICZ, avenue François Sebrechts 56 / B14, 1081 BRUSSELS (KOEKELBERG), Tf +32. (0) 2.354.0217
Genia JAKUBOWICZ, avenue du Globe 41 / B46, 1190 BRUSSELS (FORÊT), Tf +32. (0) 2,203.1636.
Perhaps you could start by asking those people, by letter or by phone, if they are descendants of known people of your family . . you might be once lucky, why not ?
Also try Facebook or Linked-In, and so on, with these two names. Sometimes this has been proved as successful for me, so I'm then putting a message on their page with hoping a response will ever come, sometimes months later (because we're no « friends » or not yet linked with each other. There's for example already one Charles in BRUSSELS, I verified it. No Genia ! And then you go on as . . a new Sherlock Holmes . .