Best structure / rewrite rules to achieve the following url

My gut feeling is that you want to use custom taxonomies for part of this. So client-name would be one taxonomy (flat), version would be another taxonomy (also flat). Wireframes might a post type or a taxonomy, not sure what makes sense for you. A little googling led me to the following links, that you might be able to shake and bake into something useful.

I agree that pages and sub-pages would be a cumbersome way to achieve this, since you’d need to recreate the tree structure for every client. With custom taxonomies, you’d create the structures once and re-use them.