Given the penchant for largely undocumented binary globs in the file, I should imagine that this should be possible - because by the nature of being undocumented, they are not part of the standard so a document which has an 'invalid' binary glob would still be conforming to the OOXML standard.
Anyways, it is not a disaster - I am sure that there must be plenty of ISO standards which are defunct or rarely used because they are irrelevant or unimplementable. OOXML is likely to become another one. Where people fail to specify exactly which ISO document standard, that is their own problem. Supporters of ODF should continue doing what they were doing - refining their specification and making it good and stop wasting time trying to berate their competitor.
Besides, now you have an easy way to tell PHBs about what each of the two standards are about:
- When archiving legacy documents, use ISO/IEC DIS 29500, Information technology – Office Open XML.
- For all new documents, use ISO/IEC 26300 Information technology - Open Document Format.