DND is a slightly different animal than merely a status, which SIMPLE can accomplish. More specifically, it's a state of the UA that affects behavior. In the real world, this is usually implemented by handsets by by returning a 480 Temporarily Unavailable, or in the case of a system acting as a B2BUA, this usually works in concert with SIMPLE to convey user status and the local PBX returns a 480 on their behalf in the first leg of the INVITE. For example, this is how the 3Com VCX does things, and Adtran PBX's.
In a nutshell, SIMPLE could only say you are unavailable, not actually effect it.
To the best of my knowledge this is no standard, and if there is, I have not seen it used. Caveat: my experience is limited to the 3Com, Aastra, and Adtran systems.