I'm sorry about digging up such an old post, but I am slightly confused about the status of support for this router. Several quick searches have yielded conflicting results so I thought it prudent to inquire here.
The hardware in the ASUS RT-AC66U is as follows:
2.4 GHz Radio: BCM4331
5 GHz Radio: BCM4360
This device is a BCMA MIPS SoC. As mentioned above, it appears there was initial support added for the device in changeset 37996; however given the recent split of broadcom subtargets it appears the router should at least boot under Barrier Breaker with Ethernet. However, because the ctf.ko module is closed source, no QoS or advanced firewall rules will be available and routing may be as slow as 130 Mb/s on the 1000 Mb/s hardware.
In terms of wifi, it appears that the b43, and b43legacy packages will not have support for either of these radios. It does not appear that even the broadcom-wl package or brcm80211 packages includes support for the radios either.
Does this summary jive with the current state of things as known by you guys?