>I have a cmRuleCustom in a CDE. Is there a practical way to use this same cmRuleCustom in another CDE without just copying it there?

This is the point about the rule classes. They are re-useable. What you need to do here is save the Rule in question as a class and then remove it from the CDE. Then, all you do is drop an instance of your new Rule class in any CDE you want. In this way, you can make generic code changes in your rule class that will affect every CDE in which there is an instance of that class. If there are specific changes that you require in a particular CDE, then you can update the rule class instance in the CDE itself. If you feel that the changes that you may make should effect all instances of that rule class, then you go back and update the rule class itself.

HTH