Custom error message on groups in complex field

Hello:
I am creating a calendar system which will allow a person to allow multiple date-time segments for a single event. I have a complex type defined, but I would like to declare custom validation for each group in the complex type’s array.

I’m fairly comfortable with using Backbone to override the validate() function of an individual field. I want to know how I can extend the type of each group in a complex type and override the validate() or isValid() methods.