Is there a way to format a MusicXML file to include arbitrary text, e.g. at the bottom of a sheet? I'm trying to write additional verses to a song without having to show them underneath the notes.
For practical purposes, the question is rather "does the program that will export the XML file deal with this sort of thing the same way as the program that imports it". That tends to make the realistic answer "no".
For example, Finale (which is pretty closely linked with the inventor of MusicXML) doesn't understand the notion of "Lyrics at the end of a song" as such, before you even get to the question of what it does with MusicXML. You can have an arbitrary block of text, attached either to a particular measure in the score, or to a fixed location on a specified page, and that's as much semantic information as Finale knows about them - i.e. nothing.
Both these types of text will be exported to MusicXML, but....
Measure-attached text is exported in a
words item attached to a staff and a measure, but there is no means for the importing program to know what it is semantically. It will just be positioned relative to wherever that measure ends up on the page after reformatting.
Page-attached text appears as
credit-words within in a
credit item associated with a position on a particular page. But that makes little sense for lyrics, because when you import the file in a different program that reformats the music, most likely the text will be in the wrong place on the wrong page.
The previous two paragraphs only apply to the latest full version of Finale - i.e. not necessarily to earlier versions, or to "Finale Lite" software like PrintMusic or Notepad etc. Other notation apps may handle the text differently, or not at all.
In practice, I would just cut-and paste the extra verses of lyrics between the two applications as text (and hope that at least some of the formatting, like the line breaks, survived!) and not bother to pick a fight with XML over them. Most likely there will be plenty of other things that you have to fix up manually as well!
I am posting a simpler answer than above, straight from MusicXML's designer. Tl;dr: The only type of arbitrary text (i.e. unattached to music elements) is called a "credit" element. That's a design choice. Regarding additional lyric frames, the MusicXML designer explicitly thinks it's "unfriendly" to singers, and hence not supported.