seuseradd is a good solution to this problem. It calls useradd (so /etc/default/useradd will be used in the regular manner), and it then does SE Linux stuff afterwards. I think that it is OK to have scripts that add system users continue to run as before, and have useradd work for adding user_u users, but require seuseradd for adding SE users.
Personally I feel an option to useradd would be a better solution in this case. It's only natural for people to look in the man page when a command doesn't do what's expected and an option will be much more obvious, and more logical, than a reference to separate command.