Since the two approaches I tried in my blog post don't work in CMS 5 I would try the following.
1. Analyze the html markup that is rendered when editing a page and see how the "Mark page as changed" is rendered.
2. Create a custom property that injects som javascript into the editpage which checks the "Mark page as changed".
3. Fix the new customproperty so it doesn't render any input controls itselfs so i'ts invisible for the editor.
Br
Per