Answer
One workaround that's worked, though acknowledged by its own author as more of a hack than a clean solution: create a second custom field on the same Custom Logical Unit that itself retrieves the related ID via a reference-type lookup. Doing this generates a 'Get' method in the LU's own generated custom API for that ID value, which you can then call from elsewhere — such as your event's PL/SQL — to retrieve the ID, and from there look up whatever additional related data you actually need. It's indirect, but it sidesteps the restriction that a plain read-only custom attribute alone can't call arbitrary API Get methods that require an ID you don't yet have.