Generally I think DataObjectManager is a great adition to Silverstripe and have used it a number of times now.
I have recently used it to manage sets of linked sitetreepages and use the SimpleTreeDropdownField which works beautifully. However I would like to make this a required field. I can't see any way to add a validator for this form. I would really appreciate it if someone could tell me if it is possible and if so how?