canonical MIME headers

I've been looking through the various MIME specifications looking for a
specific reference to a canonical form for MIME headers.  The canonical
encoding model in 2049 is not quite what I'm looking for.  Hoping I'm
simply overlooking it here's what I am looking for.

Suppose an external application wanted to digitally sign a MIME body
part, which would include its headers and content.  What it needs is a
unique and unambiguous representation of the headers.

The line terminator issue is covered by the canonical encoding model but
there's no discussion of issues like downcasing case insensitive values,
unfolding lines, collapsing repeating whitespace, ordering parameters,
etc.

Is this discussed anywhere?

Thanks,

Jim