I got a basic version working. See this gist. After I found out that os.symlink doesn't worked. I tried it with hard links, os.link exists and executes.
If I do it in pythonista shell it works as expected, that when I link to file(s) and change any of them the content of the other file(s) gets also updated.
For the files in the editor the hard links don't seem to work. Perhaps some caches in the app that come in effect here?
So I combined the suggestion from @omz with my idea and this works :-)
The only drawback is on touch more, because I need to execute the file. Is the an auto run option in pythonista?
It was fun to do this, and awesome that pythonista allows this :-)