According to [0] there is. But I think sticking to lowercase usernames isn't such a big restriction. I think adding a check that the username is lowercase and notifying the user on guix system reconfigure is just as good a solution IMO.
[0] http://stackoverflow.com/questions/6949667/what-are-the-real-rules-for-linux-usernames-on-centos-6-and-rhel-6
