I agree with Massimiliano Ziccardi description. I went with the approach 1, I have the script in JavaScript to do that. The script is at the moment configurable through json file https://github.com/wojta/xamarin-showcase-template/blob/5038f91741fce6b44ff3e48e56ba3cbf0011e547/config.json.
I have another fly to be killed with it and it's related to that version bumping. Versions are written on multiple places in multiple .csproj files. I plan to use the same approach of editing XML files and replacing versions there.
Ok, I know it seems as over-engineering.... But I think that spending more time on having script to automate these things can speed up developement. And note that with the approach 2 it's neccessary to re-run publish script again and I think that NuGet versions needs to be bumped too for this to work (so project loads new dependencies).