An update on MUA/browser capabilities:
Kmail does seem to be able to handle IMAP URIs including those
that specify a particular message; via the File -> Open -> Network
dialog, a URI can be typed or pasted, and Kmail will open the
specified message. I suspect that it wouldn't be terribly
difficult to modify Kmail to recognize IMAP URIs and open any
specified message in a new window, but as of now that capability
doesn't exist.
I can't find any similar capability in Mozilla, Evolution, or Sylpheed.