Yes, it works for CONCEALed users as well.
 
On Thu, 16 Nov 1995 01:21:14 -0600 Ingrid H. Shafer said:
>I haven't determined if this works for concealed subscribers, but you
>can find out who is nomail on your list by the following command:
>q LISTNAME with nomail for *@*
>
>Ingrid Shafer ([log in to unmask])