I do not think you can serialize without escaping. Highlighting DataPower, for example. "disable-output-escaping" only affects the OUTPUT context, not the "embedded" XML ...
I think you should treat it as binary data and get the “untouched” XML file from / object / message / node ().
Since the binary node is not "xml", you can encode it directly:
<xsl:variable name="xmlInBase64" select="dp:binary-encode(/object/message/node())"/>
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