I like this idea. Recently when one of the regions in my duchy was being taken over, I wished I had this option since I was in a position to thwart the TO militarily, I could have possibly made it a little harder for the conquerors without punishing my other regions.
Not only is that not the point of the proposal, I'm also pretty sure it woudn't have worked. The tax rate that counts in these situations is the rate the Lord imposes on the commoners. The tax that the Duke imposes on the Lords is transparent for the commoners, it won't make them happier if it's lower.
As for the request itself, I like it!
