Can I use the Rules module to send mass emails to authors of selected nodes?
If you want to use rules for that, you would have to create your own event: when you add a CCK field. You would then have to write some code to trigger then event when a new CCK field is added, and supply rules with the nodes of that type or whatever data you would need in your rule.

My guess is that the amount of custom coding needed for this to work, would be more than what is needed to just do this yourself in a custom module, with hook_mail. Also you might have problems configuring the rule to do what you need. I haven't tried using an array of data types in rules, so that might make things more complicated for you as well.