I've taken the Red Hat? init scripted posted at http://reductivelabs.com/trac/puppet/wiki/UsingMongrel and merged it with the existing stock version. Essentially, it adds support for two new settings in /etc/sysconfig/puppetmaster:
PUPPETMASTER_SERVER_TYPE This can be set to WEBBRICK (the default) or MONGREL. If set to MONGREL, it will launch multiple instances of puppetmasterd with --servertype mongrel.
PUPPETMASTER_PORTS A list of ports. If you're using the mongrel server, the script will start up an instance of puppetmasterd for each port in the list.
If you don't add either of those options to your /etc/sysconfig/puppetmaster file, the default will be to behave exactly as the current script does, so users can safely upgrade to this script without risk of breaking existing behavior.