A Hero deployed with a unit can add extra attacks and a few other things, but I don't see a way that it might affect the quality of its unit for morale or fear tests.
Would it fit within this system to have a special rule that a hero could affect its unit's quality for these tests? Sorta like Prowess for high elves, but general, and applied purely to help a lower quality unit do better than it would otherwise. I know these rules need to be sparing with special rules (or any rules), but I was expecting something like this to be standard. But I'm not familiar with the specific rules these games are inspired by.
I am thinking of the future rules as well as fantasy.
There's balance issues when doing something like that, in that it's hard to predict what units the hero joins and the associated costs. Anyway I've been thinking about ways to go around it but it's all WIP so maybe one day you'll get what you're looking for.