Guys, I've successfully built the kernel which is identical to the one supplied by ASUS. I've not tried to build the firmware on top of it, but I think it will work fine. In the mean time I will try this and then I will post the patch against linksys 1.42.3 wrt54g kernel sources, so everyone will be able to build the right kernel.
Nevertheless, we still can't rebuild all the remaining stuff - such as httpd. One of the possible solution will be using wrt54g router software with an addition of the usb related stuff.