Wow, it does work, but that's goofy. Hex ASCII code for '%' is 25 - so that can't be a coincidence.
Thing is, if I use setDataURL to read an xml file in, numberSuffix='%' works.
But if I use setDataXML to set the xml to a value inserted into a jsp, numberSuffix='%' does not work (but the goofy workaround seems to. oh well, whatever works.)