OK, I've been playing around with this a bit more, and come up with what I think is a more elegent solution.
The biggest drawback with the approach of using an Album value for all files seemed to be maintenance. I'm adding stuff into the library at a pretty constant rate, and I didn't want to add yet another step into the process of getting files properly tagged. Most titles in my collection are single disk or single file, so it seemed logical that I should have to do nothing more than I do already to deal with such titles. But I wanted an approach that treated individual disks and sets in the same way. I've read up on various uses of the series/season field, but this seemed to have similar issues to using the Disk no field. So.....
I use the Series field to mark each disk or file with the name I want to give to the "set". For Example...
30 Rock, Series 1, has 3 disks, each with it's own unique entry in the database.
For the 3 entries, I set the "series" tag to 30 Rock, Series 1
I then have a new calculated field, called Title, which checks the series field for a value. If set, it uses it, otherwise it uses the Name field.
So my 30 Rock disks above, would all have a title of 30 Rock, Series 1 (taken from the Series tag), but a single disk entry would have a Title field containing the Name tag.
So If I now have a query using the Title tag, 30 Rock, Series 1 has a single entry (with multiphle files within it).
I can now group disks/files however I want, e.g. all the Godfather films together, replicating or creating "box-set" structures as necessary.
Any thoughts?