I don't think it's possible, unfortunately.
If you are to process the output file, you may be better off exporting a SAS Dataset first, and only convert it to XML in your post-processing code after you have modified the data like you needed to do.
Hint: under-the-hood (inside built-in sas ma export scripts), they use "libname ... xmlv2" to assign an XML-backed library and simply "copy" a table there. Doesn't seem too bad, actually.
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Dmitriy Alergant, Tier One Analytics