Hi,
Lets say I create an extended module like "EventRegistration" to extend the "CalendarEvent" module.
I perform a static $hide_ancestor = 'CalendarEvent'; to hide the original, but how can I change the name of "EventRegistration" to something else without renaming the class? Even better, to give it the name in the CMS - "CalendarEvent".

Is this possible?

Or actually, I really just want to add an onAfterWrite() event to some particular modules like EventCalendar without having to hack the modules code? How would I do that?

Are you meaning in the create type XX dropdown?? If so then you can declare the add action on the class

http://doc.silverstripe.org/upgrading:2.4.0#removed_sitetreeadd_action