Restrict Entry choices just to entries with a URL

[gbowne-quickbase]

We have realized if an author links to an entry that doesn't have a URL, we've gotten errors when, obviously, trying to get the URL of that item.
It would be great to restrict the pick list to entries with URLs, AND throw an error if someone tries to save a page where the entry no longer has a URL. In the instance that someone has a previous entry and then the developers have removed the entry URL from the section!

I know we can choose which sections are available for entry choices, but if there is a lot of content and many fields, this means lots of updates to every field.