Doesn't seem possible from the Network Settings applet (only choices are no proxy and set proxy IPaddr). It may be possible to specify it by hand editing /etc/pcmcia/wlan-ng.opts, but I don't know what the syntax would be, if it is supported. You may want to search around to see how it is done for other Linux wlan implementations and see if the same syntax works on the Zaurus. Sorry, I couldn't be more help...
