Scott Wiersdorf (www.perlcode.org) wrote a Perl module to interface with procmailrc files. The name of the module is Mail::Procmailrc (available from CPAN). It comes with a simple CGI (unpolished, insecure, but useful as a proof-of-concept) to allow you to edit procmailrc files from a web interface. You can see an example of the script at:
http://perlcode.org/cgi-bin/procmail.cgi
With little effort, it could be modified to allow multiple recipes, header flag modifications, templatized (separate the HTML from the code itself), etc.
The module (not the CGI) has been used in production environments with thousands of users for almost a year, and Scott says he is comfortable with its stability.