Interesting idea.
Currently, it's so damn difficult to get to apply ducal taxes, though. Most dukes don't ask for them, and lords can easily change ducal allegiance with extreme ease if ever you ask for more than they care to part with. And it's usually impossible to justify as you get more gold than any of them, too.
Partly right now there is little need to impose the tax, thus it is often easy to find a Duchy that will offer a 0% tax on your region. By making the regional taxes more important to the Duke, we should see proper competition in regards to these tax rates, creating another consideration as a Lord when deciding what Duchy to join. I would also imagine that those regions that can't switch to another Duchy may become the target of more aggressive taxes. If everything works as Vellos imagines, then suddenly Duke need to do much more for region lords in order to keep them and preserve their Duchy Taxes, which would become a much larger part of their income then it currently is.
