Hello James
Searching the MusicXML specification:
I use to select the Developer-section, First I search the alphabetic index:
http://www.musicxml.com/for-developers/Alphabetic index (Click Alphabetic Index):
Note that only 25 per page is shown, change the number 25 to "all"
I search for "time signature" and finds:
"beats attributes.mod Element 1.0 Time signature numerator"
Now I check the DTD (click on DTD) and select section Attributes
(Click "Attributes Elements" and get "attributes.mod"). Here I read:
"Multiple pairs of beats and beat-type elements are used for
composite time signatures with multiple denominators, such
as 2/4 + 3/8. A composite such as 3+2/8 requires only one
beats/beat-type pair.
....."
The text continues with the definition of <time>.
Then I would usually like to see an example and checks the Lilypond test suite,
finds example 11c:
<time>
<beats>3+2</beats>
<beat-type>8</beat-type>
</time>
Shouldn't that be sufficient or did I misunderstand you?
Kind regards
Mogens