Dry or toast the multi grain to start off with a lower moisture. I would coat the multi grain ball, if I was you with a solution of gum Arabic or Gum Acacia. The coating gums gives the chocolate coverture a binding surface area instead of the difficult multi grain surface.
You can preserve this coating solution so that when the moisture of the multi grain tries to attack the chocolate surface area from migration upwards or outwards, this released moisture does not only make your shelf life problematic and secondly the moisture is bad for the chocolate coverture. I would also incorporate a product PGPR, that is used in ice cream manufacture, to keep the chocolate in a good state.