How to restrict users to certain Content Types? A site that I'm working on will have lots of users managing the content. Unfortunately their workflow dictates that certain people only have access to certain content types. There isn't a method to their madness, so using roles to do this will result in a nightmare to maintain really fast.

The Content Access module will not work for this. It mentions Authors, but there doesn't seem to be any configuration for this. And managing on a per node basis won't cut it.

I'm thinking the most logical way would be checkboxes on the edit user account page for admins to set. This should limit them to creating and updating content that they have been explicitly allowed to. And the admin menu should only show the content types they were assigned to.

Is there a way to implement this? Or any pointers in the right direction for the appropriate hooks?
It looks like you should give it a try to use the Group module.
You'd enable the gnode submodule, and for each group type you would define the appropriate permissions (view, edit, delete, etc) for the various Content Types.

From the details included in your question, it seems that possibly you'd only need 1 group type. And if you then would create 1 group for each of the Content Types involved, you might be close to a working solution.

More info: What's the purpose of the Group module and how to get started with it?