I think same-domain is enforced - because it's sending data down the same connection (a different domain would need a different connection). The point is that it doesn't matter where the proxy server gets the data from, if the proxy is handline HTTP/2 termination.
That's what I meant - if the HTML page (which knows which assets it will require) is not being served from the same machine as the assets will be, you cannot take advantage of HTTP/2 push.
Well, you can still utilize it. For instance, if you serve `/static.css` from your CDN; you would (should) also add `Link` headers to all same-domain assets tied to it.