GNU Emacs 24.5.1 (i686-pc-mingw32); local host is MS Windows 7; remote machines are all running variants of MS Windows.
I would swear I've used this syntax (most recently less than a month ago) but it "no longer works" (or I no longer remember.) The Windows explorer shows that the hostname, dirname, and filename exist and are accessible.
I've even forgotten the name of the access method indicated by "//hostname" so my net searches are less than useful.
I could "net use 'g:' '\hostname\dirname' /persist:yes" but I consider that a gross hack to be used only in the case I can't get the easier (for me) direct naming mechanism to work.