Theoretically, I think that should work. It (At least it used to be) specified that //site/some/path should assume the uri protocol of the current context. So if it was a link on an http page, it should assume http, same with ftp and https etc. It should work sorta like how a leading slash assumes the current site context.

This was back before the Web became the one true way and is the reason it uses 2 slashes, to distinguish protocol local from site local.