with merlins 3.0.0.4.270.26b i previously used i could use ipv6 6in4 with hw acceleration.
The bad news is that I tested ipv6 with 6in4 with this fork's version 11 and indeed hardware acceleration is disabled on the RT-N66U, but reason is not provided on the status page.
The good news is I may have found a workaround.
With Hurricane Electric tunnel broker, I did the following
On the IPv6 Configuration page:
Connection type= "Tunnel 6rd"
DHCP Option = Disable
IPv6 Prefix= <From HE tunnel details, the IPv6 routed prefix for /48 without the /48 ending>
IPv6 Prefix Length=48
IPv4 Border Router=<From HE, the Server IPv4 address>
IPv4 Router Mask Length=24
Tunnel MTU=0
Tunnel TTL=255
IPv6 DNS Server 1 <From HE, IPv6 Cashing Nameserver>
Enable Router Advertisement=Enable
Enable DHCPv6 Server=Enable
Save and reboot.
That gave me a IPv6 Lan with /56 network (which was modified to a /64 with the radvd.postconf script I posted earlier is this thread). Configuring other parts of my network to use other /64 bits of the /56 network, and I had hardware acceleration enabled on what was a 6in4 network.
For you (if you don't make use of sub-netting the /56) you could use a /64 for the IPv6 Prefix and change the IPv4 Router Mask Length to 32 as suggested on the SixXS FAQ where they say
12 = 6rd uses proto-41 as a basis, one needs to configure the manually with the tunnel prefix as a /64 and the IPv4 address of the PoP with prefix length 32.
The link to their FAQ
https://www.sixxs.net/faq/connectivity/?faq=comparison
That leaves the question as to why ASUS disabled hardware acceleration for 6in4 but not 6rd protocol?