only problem with that is it applies to the AI as well, since the AI tends to use current demand for its production rules it might lead to an badly balanced world economy.
One solution is to write a simple event to give yourself more level 1 forts. The key line is:
ChangeStrucPool = $USA_Fort;10
obviously make the country what you need but that will add 10 more basic (pre-industrial) forts to your pool - these are the ones I ran out of as they are the only ones you can build in your colonial regions.
I think some limit is WAD to be honest. They are cheap so you can spam them and the auto-garrison then really changes the nature of colonial warfare.