So I got around the not being able to extend the class by copying:
CalendarModel.java
CalendarUtil.java
CalendarView.java
CellGridImpl.java
DateBox.java
DateChangeEvent.java
DatePicker.java
DatePickerComponent.java
DefaultCalendarView.java
DefaultMonthSelector.java
MonthSelector.java
from the gwt-user.jar and putting them in my project. Changing all private to protected. I also had to delete the CSS line call to dayIsWeekend() because it would overwrite any style I added. So all weekdays looked perfect, but weekends were un-affected.
Not the prettiest of solutions, but it works.