At this current time the only way to allow channels 12,13,14 is to make your own custom firmware

Very simple - once you get all the stuff installed

Do a “make menuconfig”

Goto Kernel Modules → Wireless Drivers

Highlight kmod-ath and hit enter, Configuration and enter

Choose this setting:

[*] Force Atheros drivers to respect the user's regdomain settings

I don't see an option to enable this for Broadcom, but I only am compiling for my Atheros devices

The above is for OpenWRT images.

For building your own Gargoyle images, follow the Documentation section for Developers - clone the git, setup build environment, then do – make custom – and you can follow the above to goto Kernel Modules → Wireless Drivers, etc.

Even easier - you can edit the config.in files under the target directories and set this option: CONFIG_ATH_USER_REGD=y

see also