I wanted to complile the WIFI driver for the Pogoplug Pro because the drivers which were provided in the Enable WIFI thread didn't work. So it was a similar task compared to your needs but not a driver for a USB wifi device.
I got the kernel source code directly from git. Afterwards I rolled back to the commit of August. So I got access to the .config.old configuration file form the 14th of June. It contains already a PCI configuration. I comipled everything under Debian on the PogoplugPro. At the end I comipled the wireless driver and it works. It fits to the kernel of June 2012 which is provided in the thread
http://archlinuxarm.org/forum/viewtopic.php?f=29&t=2453. I provided the compiled WIFI driver for the Pro device in the following thread for download:
http://archlinuxarm.org/forum/viewtopic.php?f=29&t=2031.
The difference to your proceeding is that I didn't take the current kernel configuration from GIT but the configuration from ~June 2012. By this procedure I was able to compile the WIFI driver and also some RAID kernel modules which fit exactly to the official kernel from WarheadsSE. I didn't compile it on Archlinux but on Debian on the Pogoplug Pro.