I am not the authority on the IETF MIME registration proceedure,
I've just observed it in practice and have read the relevant RFCs.
I don't imply that these answers are authoritative.
I believe that the registrant for a "vnd.xxx-" designation
should logically be the owner of the "xxx-" trademark, and that
trying to register data types in the name of someone else is
a bad idea.
The type "application/auto-sense" doesn't make sense to me.
MIME types designate the type of enclosed content. If you don't
know the type, you can supply "application/octet-stream", if
you, for example, know absolutely nothing about the data type.
However, "application/unknown-print-file" might be a type for
designating "this is a file intended for shipping to an autosense
printer, but it isn't just some arbitrary bucket of bits."
As for whether you can reference a separate appendix of the
IPP model document, I think the requirements for having a public
specification are fairly generic.