Uh... building outposts costs workers... (which cost unit support as they walk to the edge...) That doesn't seem too good.
Then again the idea is still very interesting, at least nobody'll be able to walk into your territory without it being war. You can now sign as many RoPs as you like! (They give you good money too in the Emperor and below levels.)
Note: you can also TRAP units in your territory by blocking the exits from your empire. You can now destroy them at your leisure (i.e. whenever you feel like declaring war on them.)
The AI's routing methods really suck. So if they want to go somewhere they search for the most direct route. But if the way is blocked, they then search for a way around. It's not smart..., that's why galleys turn around sometimes; they need to go all the way around the continent in order to get where they want to go since you happen to be blocking one of the places where their boat needs to end its turn on.
What I mean by the above is that, trapped units will stand still since there are NO ways to reach their desired destination.
D0MINATRIX is still right though, if you're going to have fast units anyway, (you need a standing army for most games), you may as well use them to hinder unwanted settler pairs trying to cross your territory.
I think it's a good idea, but it's still a waste of workers, making it unfeasible. Try using barricades, it gets the same message across to you. (You may still ask leave or declare.) However, this way doesn't really allow you to use RoPs if you're looking to have first strike when you're being attacked (since most people don't break treaties).