I'm generating RTF (rich text) output using a UTF-8 XML source. For high-ascii values, my XML source is double-byte and does not play well within a pure RTF enviornment, and instead renders as arbitrary low-order ascii. I do not have control over the XML source. I can certainly play with the XSLT to try to translate the characters or to add appropriate new RTF controls to convert the double-byte ascii into something else.

Your output file is most probably encoded in UTF-8 too. This has
nothing to do with the encoding of the XML source file, it is
just the default.
Declare the output encoding you want to use and the output method
(it should be text, rather than the default XML):
 <xsl:output method="text" encoding="ISO-8859-9"/>