I’m thinking that a work-around could be to place a transcluded ![[header1]] at the top of your notes and ![[footer1]] at the bottom. Then, one could easily change the header or footer by changing these two files.
I don’t know how to easily add the two transclusions to a big batch of files other than using python or such, but there may be a plugin to batch edit like this(?).
Wouldn’t that then add those files as stand alone notes in the left navigation? I had this idea and wanted to do this for several things but the content won’t show up unless those files are published too. Not sure if there is a way to just hide them from the navigation.