Setting prefered ipv6 source address

[Daniel]

Hello,

I'm using a gif0 tunnel for my ipv6 connectivity (he). The setup in 
rc.conf
works fine, with an additional aliased IP. I would like this IP (which I
control the reverse dns for!) to be preferred as source addr, which I am 
able
to do with:

  ifconfig gif0 inet6 $ipv6tunnelendpoint deprecated

(This is apparently undocumented, and I found out how to do it by knowing 
about
preferred_lft of iproute2 on Linux. s/deprecated/pltime 0/ also works).

But how can I nicely squeeze this line into rc.conf, along with the tunnel
setup?