If you truly need it on every screen without fail, then UserWorkForm is the place to add it. Keep in mind this will impact every portal - including Designer Studio.
I don't think that adding it to individual portal harnesses is too much of an ask, and is in my opinion the safer location. I have never seen an application with more than 3-4 individual portals, and you typically don't want developers creating them on a regular basis in your application. Any new portal should be thoroughly vetted to ensure it's necessary and planned out before build.
I wanted to make this js available through out the application, so that I can call the script in them from anywhere in the application. From a button click using runscript, from a custom html rule and so on..