[Sorry, submitted it prematurely.] Currently, in order to specify the allowed SELinux user roles, one has to install the policy-sources package and edit the /etc/security/selinux/src/policy/users file manually. IMHO it may be a good idea if s-c-u knew to do it.
I just took a look at seuserx ("seuser -X") from the setools package and it appears to me that the functionality of that code should be merged into s-c-u (or s-c-u functionality should be merged into seuserx). These might need to stay separate since some folks will refuse to run selinx (unfortunately).
Oops ... spoke too soon: s-c-u DOES HAVE support for selinux now (the support is not correct but that is another bug report). This bug should be closed
closing as per last comment