I decided to look around on the net, cause I was starting to doubt a bit, and I found an article
which indeed says that the interior is protected (the article says fourniture, but I assume they mean everthing inside). Warning though, it's in Dutch.
There was indeed a court case, Brussels capital region wanted to protect the fourniture and the family went to court, but court disagreed with them. I assume the brussel capital region protected the interior in preparation for world heritage inscription.
If you're ever able to go inside, I don't know. The house is still privatly owned, and I got the feeling they don't like all this attention. Perhaps one day when they sell it...or move out...
I was also thinking about the belgian house of Le Corbusier on the tentative list. It's also privatly owned (even by a belgian celebrity if I'm not mistaken) and can only be visited from the outside.