While you could fill it in with ifconfig, it's actually easier just to assign the subnet through the System Preferences by setting your IPv6 configuration to manual and entering it there.
In my case, I allocated a /48 to my HE tunnel and used that to setup the tunnel on GIF0, and then picked an address in my assigned /64 to put on my en0 interface.
So basically, an IP address in my routed /48 gets assigned to gif0 using:
ifconfig gif0 inet6 2001:470:b110::bdc:caff/48 alias
...in addition to the usual commands to setup the tunnel.
I then assigned one of the addresses from my routed /64 to en0 via System Preferences -- select en0, choose "Advanced" and select "Configure IPv6: Manually" and enter the following info:
Router: The Server IPv6 address from your tunnel (same as your default route set by the route command when setting up the gif0 interface)
IPv6 Address: Your /64 prefix and a node on your network (ie, 2001:470:1d:403::1)
Prefix Length: 64
Once that's setup, you should be able to just fire up rtadvd by entering "rtadvd en0" without worrying about the rtadvd.conf file, as the daemon will pick up the network configuration from en0 automatically and advertise addresses within that subnet. Other machines on your network should autoconfigure with addresses in the 2001:xxxx:xxxx:xxxx::/64 subnet and pick up the default route via your iMac.
Note that you may also need to set net.inet6.ip6.accept_rtadv to 1 on the remote computers for them to pick up the stateless autoconfiguration from your iMac and obtain addresses on the routed /64 network.
In the end, my ifconfig for gif0 and en0 reads as follows:
gif0: flags=8051<UP,POINTOPOINT,RUNNING,MULTICAST> mtu 1280
tunnel inet 192.168.101.101 --> 216.66.38.58
inet6 2001:470:b110::bdc:caff prefixlen 48
inet6 2001:470:1c:403::2 --> 2001:470:1c:403::1 prefixlen 128
en0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1280
ether 00:23:df:91:9e:92
inet6 fe80::223:dfff:fe99:9f93%en0 prefixlen 64 scopeid 0x4
inet6 2001:470:1d:403::1 prefixlen 64 autoconf
inet 192.168.101.101 netmask 0xffffff00 broadcast 192.168.101.255
media: autoselect (1000baseT <full-duplex,flow-control>)
status: active