How to make custom footer section stick to bottom of page in Pages 5?
This is currently not possible.

You can have it at page 1 and page 2 using a normal footer as you're probably aware, but not automatically move to the last page of a document. You can do it manually as you're also probably aware by using the section inspector, but that's still not automatic and would require you to delete it from page 1 and then add it back to page 2.

You can notify Apple that you would like this feature implemented using their Pages feedback page:


http://www.apple.com/feedback/pages.html