The other day I realized that you can use a collection of InsertActions to set defaults in a Group with repeating items.
Unfortunately the action is only triggered upon a Save. But in combination with a HideAction for the Group it works OK.
For those of you who want to find out the details by themselves: add the GroupOID plus ordinal to the DestinationProperty of the InsertAction.
For others: on http://www.trialdatasolutions.com/tds/howto/defaultrepeatingitems.jsp
you can find step-by-step instructions.