but it's far more likely that it doesn't support --stdin that that it does.

Well given that it is valid for passwd(1) on RH, and RH is the most popular Linux, and RH has 50% market share according to netcraft, which is 3x Debian I would simply have to disagree with that premise. Linux is also about 2:1 Unix and 50% of Unix is Solaris so RH is 2:1 Solaris AOTBE.

[root@devel3 bayes_cache]# uname -a
Linux devel3.blah.org 2.4.18-3smp #1 SMP Thu Apr 18 07:27:31 EDT 2002 
+i686 unknown
[root@devel3 bayes_cache]# man passwd
PASSWD(1)     User utilities      PASSWD(1)

NAME
       passwd - update a user's authentication tokens(s)

SYNOPSIS
       passwd [-k] [-l] [-u [-f]] [-d] [-S] [username]

DESCRIPTION
       Passwd is used to update a user's authentication token(s).
[snip]

       --stdin
              This option is used to indicate that passwd should read 
+the new password 
              from standard  input,  which  can  be  a pipe.
[download]