Composer repository URLs can’t use UNC paths?

It would seem that when using Composer on Windows, one cannot use UNC paths as repository URLs. That is to say, the following doesn’t work:

{
  "repositories": [
    {
      "type": "vcs",
      "url": "//SERVER/Share/project.git"
    },
  ]
}

Instead, the share must be mapped and the mapped drive letter used:

"url": "G:/project.git"

This does, of course, have the stupid side effect of forcing everyone who uses the composer.json in question to map the share to the same drive letter! However, this is hardly the most significant of the miseries that one must live through when developing software on Windows…