ha, I'm working on a dev-side version similar to this (mostly just for me, but hopefully publishable). I opted for an entirely pre-deplopyment build tool, where you just put

  <custom-tag custom-param="value"></custom-tag>

in your HTML, run the build and it outputs the filled in file somewhere else. I know its functionality is very similar to many web frameworks (e.g. React, handlebars) but it does one thing.