I think we just need to make sure that page_content_model is set to flow-board in the database for the pages, and $wgContentHandlerUseDB is true.
Then, I think it should Just Work, and we can take out this code.
We might also want to write a script to allow enabling a new page (depending on product decisions for enable/disable UI). It can be done with flow-create-board, but not easily for a one-off.
We can now also get rid of $wgFlowOccupyNamespaces, since now that all subpages are Flow by default there is no meaningful difference between the two.