It's going to be really difficult to heal that scar over while keeping the tree in a bonsai pot with any technique or treatment. If you don't eventually want a hollow trunk or large deadwood feature it needs to go in the ground.
My experience with wood hardeners on a variety of species leads me to believe that hardener doesn't seem to have a really noticeable effect. The largest factor is the inherent strength in the wood itself. Ficus, Bougie and other softwood trees will simply never hold their deadwood for ever (that might be 5 years or it could be 50+). I would however, recommend torching dead wood and applying lime sulfur.
Also, there is another thing to consider, you want the dead wood to be able to breathe. For example, if you paint every single face of a fence slat it will rot out in no time. Allowing humidity to be able to escape will keep the wood a lot longer. Let me plainly state I have no empirical evidence for this in the context of bonsai - but I think it's worth further investigation.