Event registrations: possible to set at application or even group level?

We don't currently expose settings for default event parameters. 

One solution might be to enhance the "Calendar Event - Add/Edit Event" widget with configuration options for default event settings. Then for the group in question, you can edit the widget configuration with your desired settings, and back it with appropriate parameters provided to the create commands in the widget. This isn't the ideal location for this logic, since it only applies to the one widget, but it might do as a stopgap.

You could even create a contextual Management Panel to do this, this change would be even more extensive as it would require creating storage backing for the settings and modifying the add/edit widget to retrieve them.

Incorporating this functionality for future versions as a management panel would be a great candidate for our Ideas and Improvements page.