You're right, BLOCKQUOTE sounds adequate to me.
As for the URL's, you're also right:
"when interpreting relative URLs in a MIME multipart, the message
should be regarded as a directory, with each part being a file named
by its Content-ID. Multipart content-types are represented as directories".
In this way, a reference to <a href="content-1(_at_)message"> is perfectly
reasonable and unambiguous.
That strikes me as very reasonable, but should be spelled out in a
published spec for text/html. What are the file naming conventions for
URL's in general? That is, can this be distinguished from a local file
reference by absolute path names? Should the latter be permitted inside
a MIME multipart at all? I suspect not, but we need a clear spec.
You can also do
<a href=multipart1(_at_)msg/content-1(_dot_)2(_at_)msg> to refer to a
specific portion
of an adjoining multipart, or even <a href=../content-2(_at_)msg> to refer
to a body part that is adjoining the parent multipart.
Given that Content-ID values are supposed to be globally unique, this is
probably not necessary, and is therefore (to my mind) unnecessary
complexity... -- Nathaniel