Not in the decree, or any of the past decrees concerning the heraldry of the royal family.
I am not sure if there is (still) an official regulation granting wives, in general, the right to bear the arms of their husbands.
Not on the basis of the decree. Article 4 explicitly applies only to descendants of King Leopold I:
Art. 4.
Les autres Princes ou Princesses de Belgique de la descendance masculine et féminine en ligne directe de Sa Majesté Léopold Ier portent l'écu des armes, en losange pour les Princesses, tel qu'il est décrit à l'article 1er, chargé d'une filière d'or. [...]
(Art. 4. The other Princes or Princesses of Belgium of the male-line and female-line descendance in direct line from His Majesty Leopold I use the shield of the arms, upon a lozenge for the Princesses, in the form in which it is described in Article 1, charged with a gold border. [...])
Note that in Benelux heraldry, a lozenge is the traditional form for the arms of an unmarried woman (in today's circumstances, it is also in use for the arms of women who are armigerous in their own right and not by marriage), whereas an oval is the traditional form for the arms of a married woman.