I get your point, but I prefer the opposite approach: all the customizations in a single plugin (html, css, js and any changes to the backend), in this way I have them on files in a single object instead of scattered in the database and above all I can version them. In this way I can activate a new magazine by recycling the previous work and reconfiguring it in one place instead of installing many plugins and configuring them all one by one.
Anyway this is a theoretical discussion, the point is that it can’t be done, I take note, I prefer to do the same thing hardcoded into headerHead.tpl which is pretty much what Custom Header does.
Thanks for the feedback