They're actually going to fix the URL problem once and for all using [Cross-Origin Server Push][1] in the upcoming Signed HTTP Exchanges standard. Here's their blogpost on that: https://amphtml.wordpress.com/2018/01/09/improving-urls-for-...
[1]: https://tools.ietf.org/html/draft-yasskin-http-origin-signed...
(Author here.) The "Signed HTTP Exchanges standard" is part of the Web Packaging standard, which is, indeed, the whole point of the article. https://github.com/WICG/webpackage