Hi Dave (cc to the list),
I am creating xml on an IBM mainframe. The code page used on mainframes
is defined as EBCDIC 0037. https://en.wikipedia.org/wiki/EBCDIC_037
As EBCDIC 0037 is limited to 256 characters, a simple mapping should suffice to convert your text to ISO-8859-1 (see the mapping table in the Wikipedia entry you referenced). It should be lightning fast, no matter if you do it in assembler or in XQuery.
Hope this helps, Christian