Feature Request: Prevent pages from being a parent

I would very appreciate an option for pages to prevent them from being a parent page. The background is, that one of my client has a very deep page hierarchy. When creating a new page, he has to scroll through about 500 pages to find the right parent. An option like this would tidy up the dropdown for setting the parent page drastically.

Approach as follows:
1. Add a Y/N-ticker in the options pane of the page editor with "can be parent"
2. Add a filter on the "parent page"-dropdown that looks for "can be parent" -> Y

Hope to find this feature in the next release :)