No joy... explicitly selecting use for initialization had the following results.
Using Attibute IS UNDEFINED:
"Value was not changed because it was equal to the old value StartDate=null" (odd, I would associate that with WAS CHANGED)
Using an Unconditional Rule:
Same as above
Using BO IS NEW:
"Action XNPBO_Review.StartDate=LAST_DAY_OF_MONTH(12) is not allowed during initialisation."