Hi Martin,
Could you please clarify, what do you mean by "default value of custom URL path"? When you create a new page do you want to check it and pre-populate by default? Or do you want to change the value for existing pages? Or do you want if admin ticks this checkbox the value was pre-populated instead of empty?
What is the main problem to solve? Why do you need it? Potentially, there are other ways to do it.