Mně to funguje i za normální maškarádou:
~ # ip a s eth0
...
26: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
inet 192.168.0.126/24 brd 192.168.0.255 scope global eth0
...
~ # ip tunnel add he-ipv6 mode sit remote 216.66.86.122 local 192.168.0.126 ttl 255
~ # ip link set he-ipv6 up
~ # ip addr add 2001:470:6e:834::2/64 dev he-ipv6
~ # ip route add ::/0 dev he-ipv6
~ # ip -f inet6 addr
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 state UNKNOWN qlen 1
inet6 ::1/128 scope host
valid_lft forever preferred_lft forever
26: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 state UP qlen 1000
inet6 fe80::3285:a9ff:fe17:5dc8/64 scope link
valid_lft forever preferred_lft forever
34: he-ipv6@NONE: <POINTOPOINT,NOARP,UP,LOWER_UP> mtu 1480 state UNKNOWN qlen 1
inet6 2001:470:6e:834::2/64 scope global
valid_lft forever preferred_lft forever
inet6 fe80::c0a8:7e/64 scope link
valid_lft forever preferred_lft forever
~ # ping6 nix.cz
PING nix.cz(info.nix.cz (2a02:38::1001)) 56 data bytes
64 bytes from info.nix.cz (2a02:38::1001): icmp_seq=1 ttl=58 time=65.4 ms