As noted in the submission of an item to the ePub Maintenance group,
my study of the OPS/OPF/DTBook specs indicated that we made no
statement regarding the encoding of the NCX. It is possible I missed
any explicit statement to the contrary.
Regarding the intent, I'm not sure we ever discussed limiting the
encoding for NCX to only UTF8-16. It may have fallen through the
cracks.
So, it is something I hope the Maintenance group takes up and
resolves.
Regarding epubcheck, I do hope it already checks for character
encoding of all the XML documents...