hashed MAILDIR with procmail filter mode

I wonder if someone has modified procmail to support
a hashed mail directory with filter mode. Let me explain:

I'm using procmail in a special sendmail mailer definition
for POP users without UNIX accounts. This POP mailer calls 
"procmail -m rcfile $f $u" with the rcfile

# /etc/procmailrcs/rc.pop
MAILDIR=/var/pop    # Mailspool
FROM=$1             # Sender (in case we do forwarding)
DEFAULT=$2          # Recipient

MAILDIR will be very populated so I'd like to have a hashed 
MAILDIR similar to procmail's MAILSPOOLHASH. I can't use 
MAILSPOOLHASH since it only works in delivery mode (procmail -d $u).
Of course I could use an external program to spit out the
desired file name for $DEFAULT, but I would rather avoid this.
I imagine something like

MAILDIR=/var/pop    # Mailspool
MAILDIRHASH=2       # $MAILDIR/f/o/foobar
FROM=$1             # Sender (in case we do forwarding)
DEFAULT=$2          # Recipient

which a) automatically delivers to "f/o/foobar" when DEFAULT is set
to "foobar" and b) creates missing directories in MAILDIR on the fly.

Any hint on how and where to modify the procmail code for
this is welcome.


Juergen Georgi