passing username to spamc through maildrop with postfix

I'm setting up a purely virtual postfix/mysql mail server, and I wanted to implement spamassassin to filter spam. Initially I had intended to use procmail, but postfix doesn't currently support procmail for virtual users, so I had to go with maildrop. All I wanted was to enable the line:

xfilter "/usr/local/bin/spamc -u USERNAME"

and have the username changed to the recipient for the mail being processed. After stumbling through some changes in master.cf and trying different maildrop variables, I found that the easiest way to get the username to the script was simply to pass ${recipient} as a parameter to maildrop in master.cf, like this:

maildrop unix - n n - - pipe
flags=DRhu user=vmail argv=/usr/local/bin/maildrop -d ${recipient} ${recipient}

(yes, ${recipient} should be there twice.) This way, I can always pull the recipient email address directly into the maildroprc file by referencing $1. So my xfilter line became:

xfilter "/usr/local/bin/spamc -u $1"

then I just fleshed out my maildroprc script (i'd post it here but it's pretty specific to my setup, and other howtos on maildrop/postfix are available) to handle what happens if a mail is tagged as spam.