It seems that beginning with Mercurial version 3.0, it is now impossible to use purely numeric tag name :

$ hg tag 1
abort: cannot use an integer as a name
$ hg --version
Mercurial Distributed SCM (version 3.1)

But it is working on version 2.2 :

$ hg tag 1
$ hg --version
Mercurial Distributed SCM (version 2.2.2)

I don't know if there is a lot of Mercurial users out there using RMT, but this issue should be addressed. I see two possible solutions :

1. Forbid the use the Simple version generation when Mercurial is used
2. Add some kind of prefix to simple tags, like 'v\d+' instead of the current '\d+'

The problem with 2) is that it is can be a big BC break depending on the implementation.

FYI : the test suite is currently failing when ran with the "faulty" Mercurial version.