In a recent thread on the sup mailing list there was discussion of the
similarities of the recently-added sup-cmd command-line interface with
that of notmuch. And there was at least one proposal for adding some
syntax to notmuch:

On Fri, 05 Mar 2010 00:40:27 -0800 (PST), Nicolas Pouillard <nicolas.pouillard@gmail.com> wrote:
> Yes please add the is: prefix too, tag: was added to sup.

I've just added this to notmuch. So you can now search for "is:unread"
as a synonym for "tag:unread".

Have fun,

-Carl