How to do both DHCPv4 and DHCPv6 at the same time?

[Darren Pilgrim]

Comcast does IPv6 using DHCPv6 and DHCPv6-PD.  At least in 8.3-p3, the 
in-base dhclient doesn't do DHCPv6.  I installed the 
net/isc-dhcp42-client port and am successfully using as a workalike 
drop-in replacement with the following in /etc/rc.conf:

dhclient_flags="-lf /var/db/dhclient.leases.${ifn} -pf 
/var/run/${name}.${ifn}.pid -cf /usr/local/etc/dhclient.conf"
dhclient_program="/usr/local/sbin/dhclient"

According to the port's dhclient man page, I need add "-6" to 
dhclient_flags to make dhclient do DHCPv6.  The problem is the network 
scripts in the base don't differentiate between DHCPv6 and DHCPv4, so 
adding that would break my network config (or at least prevent the 
system from getting an IPv4 address).  Writing my own RC script for this 
is completely the wrong solution, so I'm sure I'm overlooking something. 
  What am I missing?