procmail to deliver in a Maildir/ for every user?

[Stephane Bortzmeyer <bortzmeyer@netaktiv.com>]

Hello,

I wish to use procmail as the system-wide delivery agent (the MTA is
Postfix) *and* to have mail delivered in qmail-style Maildir/ by
default (the POP and IMAP daemons are Courier, which only handles
Maildirs).

If I write a /etc/procmailrc:

:0
$HOME/Maildir/

it works but it even does so for the few shell users which have a
~/.procmailrc (the home procmailrc is read after, when the mail has
already been delivered). The procmailrc syntax does not allow me to
test the existence of a ~/.procmailrc. 

Is there a way to combine my wishes? Otherwise, I'll use Postfix
internal MDA (which will make it inconvenient to have system-wide
services such as mail duplicata removal) and the shell users will have
to invoke procmail from a ~/.forward.

Possible solution, untested: write a local MDA which is a very simple
shell script. It will test the existence of ~/.procmailrc and will
invoke procmail with different arguments. Ugly, I think.