Are you using boot option waitusb=5 ?
Note too that you would also need wireless_tools extension in your tce directory
Although I don't recommend running from pendrive, only because they are much slower than ssd, it should work.
Example:
On my dell mini 9 using unetbootin, I boot with additional options of
waitusb=5 tce=sda1 restore=sda1 nodhcp
This is with a pre-existing tce directory on the pendrive containing:
broadcom.tcem
wireless-2.6.26.tcem
wireless_tools.tcel
Then in /opt/bootlocal.sh I have
modprobe wl
sleep 1
/usr/local/sbin/iwconfig eth1 essid "ETDialHome" key 1122334455 && udhcpc -i eth1
This should persist when done with a backup upon exit.
Note: You need full path to iwconfig
For 900A, which uses madwifil, you would have madwifi-hal.tcem instead of broadcom and your modprobe would be for ath0
Note: I typcically run netbooks by copying two files onto the ssd and adjusting the existing grub menu.
It is much faster than pendrive.