When trying to use a short repository name, I get the following error:

2015/10/08 09:40:13 Invalid namespace name (abc), only [a-z0-9_] are allowed, size between 4 and 30

Since this is not an error docker generates (I have no problems to use that name with Docker 1.7.1 when building the image manually) but rather is a constraint by the official Docker Hub Registry, this should not be enforced at plugin level. Or at least not for any other Docker Registry than the official one.