After building and a couple of tests, it works.
To install, you need a rooted Xoom, of course, and just push the file to system/bin/wpa_supplicant.
Because zipfiles don't store permission, you have to make it executable as well.
adb remount adb push system/bin/wpa_supplicant /system/bin/wpa_supplicant adb shell chmod 755 /system/bin/wpa_supplicant
adb shell chown system.wifi /data/misc/wifi/wpa_supplicant.conf
It works with my Nokia N8 with JoikuSpot; I can't tether it any other way as the N8 doesn't support the PAN Bluetooth profile, and Android doesn't support DUN.
Don't forget to backup your files, and I am not liable, etc, etc.
The code I used is here: