I agree with Peter, CMS should allow the extension of the Choice syntax
i.e. in text using the old ASN.1 rules), which gives a good pointer to the
implementors that they should not object if the they receive an extended
choice.
I also agree with John Pawling, that any extension must be controlled and
should only be done when there is a requirement not met by the current
syntax. As per Peter's comment, control of the choice can be achieved by the
normal RFC process. Also, if more control is required on S/MIME for
interoperability reasons, the S/MIME specification (not the CMS spec) could
prohibit the expansion of the choice unless a new version number is agreed.
Regards
JR