I like the direction this is heading.

Perhaps this would be better tackled by forcing the same page-type selector, but simply allowing users to choose from existing page templates there. Then you could use your custom meta box plugin of choice to control the meta boxes used for each template.

I've also seen some custom themes which have forced users to select a page template before publish a page. That way you know the user has thought about which page type they're publishing and not just accepting the defaults.