I eventually tossed out Extended WPFToolkit due to it's inability to access the checkboxes directly.
Instead I created a ComboBox and manually defined Checkboxes within it, which I access directly by name, and there able to implement a "Select All" by using it's [Checked/Unchecked[ event, and use the ComboBox SelectionChanged to show a default value that expresses what has been selected in a CSV format.
Maybe be clunky, but it gets the job done.
PS. I did not need to even bother with a DataTemplate for the ComboBox
SelectedItems="{Binding SelectedItems}"
- James Sampica