Option to prevent creation of selected content types without a parent book

I'm aware of that setting (and am using it) but it doesn't solve my problem completely.
People can still create content of the type that I have designated as the one I want to use for book 'pages' independently of a book.
I want to restrict access to the create content page of this particular content type unless they are specifically adding to a book.

Perhaps an example will help me explain...
I have a content type 'bookpage' and a content type 'book'.
'bookpage' has 'forbid creation of book main page', has no possible child content types and has only 'book' as a parent type.
'book' has 'auto create main page', has 'bookpage' as its only child type and has no possible parent types.
Additionally, I have removed 'bookpage' from the 'create content' menu, so the only link in the UI to the creation of a 'bookpage' is through the 'add a book page' link on the 'book' content type.

This works, but if someone navigates to /node/add/bookpage they can create an orphaned 'bookpage'. For me, an ideal solution would restrict access unless the 'bookpage' was being added to a valid book (i.e. the presence of '?parent=x' in the url).

I hope that clarifies what I meant.
Hopefully this isn't too specific a use case. It only makes sense if you have a separate content type to be used as a page and you do not want that type being used for anything else.

That does the trick!

That said, I did find a way to break it. Sort of.
It certainly does what I asked for, but if I stick something that doesn't actually correspond to a book in 'parent=' then it still lets you create the node.
Plus any nodes created like this don't allow you to edit them (you get "This content-type cannot be created outside a book. Creation cancelled" when you try to).

Oh nice! That works. Forbidden content types are being limited properly - I can't trick it into letting me create one outside a book.
And the depth limit setting is working great too.

EDIT: Ah, I thought I'd test it a bit more thoroughly and I think I found another problem. You can use the outline and reorder tabs to bypass both of these rules, by moving a child to be underneath another child after the initial creation.