If the portal does not have a sort order and if the portal is not filtered, the last function can be used to refer to the last related record:
Last ( portalsTableOccurrence::Field )
will return the value in Field for the last related record that contains a value.
(This function works directly off the relationship so any portal settings that change what record is displayed last will be ignored.)
Thanks Phil. Yet another perfectly answered question. As I suspected, it was an incredibly simple answer that just completely escaped my thought pattern.