Provide hook/Event to alter configuration pre build

I have a use case where I need to adjust configuration and depth based on a few situations.

I could extend the MenuBlock class to do this but I thought it made sense to provide a hook/event to do this.

I've rolled a patch for 1.5 since thats what the site I'm building is on. If the maintainers agree this would be valuable I'll reroll for the latest dev and provide the api file etc :)

In a way this is similar to what hook_menu_block_tree_alter did in D7.