How does cutting off British goods by a tariff wall help these states any more than cutting them off by an embargo?
By producing revenue for them? Obviously it is still a major trade shift, but they actually gain something this time. This is beyond any protection for local industrialists, which as others note in the thread, the French will make up, but not to the extent that the British did.
And if the Customs Union does produce any revenue (rather than just causing imports to dry up) then Napoleon has a clear incentive to steer as much of this as possible into the French Treasury rather than see it go to petty princelings or unreliable allies. So he has just as much reason as OTL to annex Holland and North Germany.
This entire thread is based upon Napoleon having a more effective economic policy, there wouldn't be any reason to annex those states since it would benefit France more to have them as client states which earn money, some of which will go to the French treasury and some of which will help those states, secure his flanks, buy local support, etc. Napoléon didn't annex states on a whim, the Netherlands and North Germany weren't annexed until 1810, and they were annexed for entirely different reasons than monetary ones.