Okay, I've worked on this some more, and I think I know what I have to do . . .
Set up more than one relationship, one each for Film Roll I.D.'s relationship with Film Roll ID1, Film Roll ID2, Film Roll ID3, etc.
Display the Edit Relationship dialog box for each relationship, and select "Allow creation of records in this table via the relationship" every time.
Create multiple portals on the Film Rolls layout, one for each relationship. This will allow listing the related Photo Set names.
Now it becomes only a design issue, how to neatly display the multiple portals on the layout.
. . . unless someone can recommend a better approach.
This sounds very much like a classic "many-to-many" situation; which requires 2 "entity" tables, with a "join" table between them, having an ID field from each of the entity tables. So a Photo set can be at multiple location/times, and that location/time can have multiple photo sets, independent of each other.