What is the canonical penalty for Catholics who get ordained as "mail order ministers"?
The Church seems to be silent on this matter, not because of its' gravity but more so because the whole purpose of canon law is to set down guidelines for Catholics in good faith to make the right choices and to act accordingly. How can one be in communion with the Church while being an ordained minister in another denominational church?
Catholics who freely get ordained as a non-denominational minister cease to be Catholics in good standing and as a result canon law will no longer hinder them because they outside the jurisdiction of the Church. They are outside the unity of the Church and are to be considered heretics and/or a schismatics. How can one serve two masters. Either he will love the first and hate the second? See: Matthew 6:24 One is Catholic or one is something else!
How Can someone claim to be a practicing Catholic and then get ordained as a non-denominational minister (such as from The Universal Life Church Ministries) makes no sense. One can not abide by what the Church teaches and then leave the fold and teach what may be contrary to Catholic theology.
The following article is found online and although it is only indirectly on this topic, many of its' points are quite prevalent to this subject: Catholic Priests Who Become Non-Catholic Ministers.