Many months ago, I privately raised the question as to why some other
method if identifying content-types (e.g., object-identifer) wasn't
used. If one went with OIDs, registration is no longer centralized.
The response I got was that this issue had been raised, argued
extensively, and decided. At this point, I am unwilling to re-open this
issue, and I suggest that you do not re-open it either.
Having said that, I think it important to keep in mind that the X-
extension mechanism can still be used for unilateral purposes. If
something is to be "standardized", you have to give it to the IANA.
Otherwise, the IANA isn't interested (and you use the X- prefix instead).
/mtr