More interestingly, I could envision an actual letter of marque being issued to a specific army, granting that army permission to loot in a specific realm, and returning some percentage of the takings to the Crown.
This would not require a declaration of war, just a lack of a peace treaty. It would also not permit any takeovers, just looting.
This sounds a lot like the scrapped new diplomacy system that allowed narrow declarations of war - such as declarations that would only allow looting, or declarations that automatically expired once a limited objective was achieved.
I liked that system except for the treaty friction part. The taxation part is really not necessary for it to work.
