I have now merged
We will release Hugo 0.21 in about a week; there are a few issues left to tangle, but we should get the above feature out in the wild, as it fills a vital gap in the Custom Output Format, esp. for Google AMP pages.
So if you can, please take it for a test drive … You don’t have to test that feature, just test that your site runs OK. If I would predict any potential issues, it may be in the live-reload department. But I think it should be all good …