I looked again at the BibX example for partdesc (which is where my
partDesc came from).

Would this -- the ability to have composite references (the specific
details of how to name elements is less important) --not better
accommodate your example Suzanne?

  <partdesc>
     <composite>
          <single type="volume">3</single>
          <partdesc>
             <single type="part">1</single>
             <single type="part">3</single>
          </partdesc>
     </composite>
   </partdesc>

This was designed to be able to get a citation like:

Some Collection Title, volume 3, parts 1, 3.

The other example was...

  <partdesc>
     <single type="chapter">1</single>
     <range type="chapter">
          <start>3</start>
          <end>5</end>
     </range>
  </partdesc>

...to get:

Some Book Title, chapters 1, 3-5.

BibX, BTW, parses all start/end extents, including page numbers
(important for formatting processing).

The above doesn't solve your interest in including titles of course.

Bruce