Thanks for the push in the right direction!
Even though it took a bit more time than I had intended, the end result is certainly worth the extra effort. Here's what I did in case it helps anybody else out there:
1) Downloaded and installed the very useful "Find Duplicates" plugin for Calibre.
2) Created a new user metadata field labeled "Version" using "Add your own columns" under Preferences.
3) Ran the plugin to locate all duplicate titles in my library.
4) For each duplicate title, I assigned text to its new "Version" metadata tag. In my case, I used an abbreviated form of the publisher's name.
5) Adjusted my Save to Disk template to add "[version]" after the book title when that tag is not empty:
{author_sort}/{series:|- | -}/{series_index:||)} {title} {#version:|[|]}/{title} - {authors}
Now, my Save to Disk output looks something like this when exporting duplicate titles:
Twain, Mark/Huck Finn [Penguin]/Huck Finn - Mark Twain.epub
Twain, Mark/Huck Finn [Random House]/Huck Finn - Mark Twain.epub
The only downside that I can see is having to manually maintain duplicate titles in the library, but since I'll rarely add titles that I already have, this shouldn't be much of an issue.
Once again...
