Earlier quoted context omitted.
If you can write a tag that references app.ab9a9a09fe9.js, presumably you can write a HTTP header that references that same file, no?
What about a static website served through nginx, for example?
Then, your _headers.j2 file should look like this: /*
Link: ; rel=preload; as=stylesheet
Which will just be replaced with the correct value, same as in your HTML template.