Yes, this case is famous and has already been discussed in the past.

They should have replaced the "zone 30" sign with a digital sign that would be active only on certain hours of the day.  Apparently they still haven’t done it.

My bet is that it should be maxspeed=30 then, on the principle that the most restrictive rule always takes precedence.

May I suggest you report the problem to the Brussels Region directly (e.g. through FixMyStreet).  It would be better than trying to figure out how to solve this impossible quest.