Now I am looking for feedback about general approach. Of course it could be somehow polished and improved to hide some boiler plate.

This part:

%{echo:%(
[ ! -e %{S:1} ] &&

Looks really clumsy. After reading the

  https://pagure.io/packaging-committee/issue/1132#comment-769233

I like

 # Script: gen_clean_tarball.sh

approach. Whether it will be special comment, macro (with support for extraction in spectool) or new tag in RPM - I do not care. The important part is that it is standalone file, which can be easily locally executed. That would ease development and debugging.

Miroslav