Forgive me if I have overlooked something but was there a custom firmware selector too? I seem to remember it linked somewhere but have forgotten where it was. Not the software to install on Ubuntu, etc to create a custom install of OpenWrt but a site like the selector we use for the pre-built images.
If you go to http://firmware-selector.openwrt.org and then type in your device, you will see the download links. Right above that section is a triangle that says "Customize installed packages" -- click that triangle and then you can build your own custom images.