Set same field value for all items mass-transitioned together?

Does anyone know any out-of-the-box way to set the same value in a field for all items mass-transitioned together? Items appear on a report; user selects items and does the mass transition. I want all items transitioned this way to have the same date/time or serial number in a field.

The next time a user does the same thing the field is a different date/time or s/n.

I can’t use the date/time of the transition because, worst-case, there may be delays by the time the last item selected is transitioned.