Your issue is legit and I raised the same a while ago :
https://github.com/bevry/docpad/issues/276
In the end, hypercubed developed the "raw" plugin available through npm :
https://npmjs.org/package/docpad-plugin-raw
Depending on what you provide as options, it can either do a cp or a rsync command. So it basically what you intend to do with a Grunt task.