Nathaniel,
my feeling is that it is doable without an URL syntax extension.
Simply say that
"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. 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.
About excerpt (would have been clearer in HTML :-):
Quoting from http://www.ncsa.uiuc.edu/General/Internet/WWW/HTMLPrimer.html#A1.4
.3:
Extended Quotations
Use the <BLOCKQUOTE> tag to include quotations in a separate block on the
screen.
Most browsers generally indent to separate it from surrounding text.
From RFC 1523:
Excerpt -- causes the affected text to be interpreted as a
textual excerpt from another source, probably a message
being responded to. Typically this will be displayed
using indentation and an alternate font, or by indenting
lines and preceding them with "> ", but such decisions are
up to the implementation.
I don't see the difference as big enough to warrant a change in HTML.
Harald A