How to disable at-boot configuration of a network interface but permit manual use of rc.d?

[Darren Pilgrim]

There are some conditions to the task given by the subject:

1: The interface must be present at boot.
2: Use of /etc/rc.d scripts to start and stop the interface is
desirable.

The first condition poses no problem, just don't include the relevant
ifconfig_ifn line in /etc/rc.conf and the interface won't be configured.
But rc.d/dhclient and rc.d/netif won't work without an ifconfig line for
the interface.

Adding the ifconfig line and then listing every interface but the one I
want configured in network_interfaces does prevent it from being
configured at boot while having an ifconfig line in rc.conf, but if I
try to use rc.d/netif to start the interface, rc.d/netif does nothing
because it tests the interface against the contents of
network_interfaces and cloned_interfaces, so the interface I left out
will be excluded.

Have I overlooked an option somewhere?