There is in fact some draft language around this kind of a mechanism to update a signature to extend the lifetime of the document by fetching a remote URL. See https://tools.ietf.org/id/draft-yasskin-http-origin-signed-r... .

Doing this on every page load breaks either user privacy (by making the origin fetch before the user clicks) or the preload performance gain itself (by blocking load while waiting for this round trip).