what i want to do with am200g:
use usb to plug usb-hub and make one usb internal with flash to install programs and one usb external to mount usb-hdd.

1 - i've recompiled firmware from GPL Source downloaded from Asus.com and added vi,wget,adduser, etc. firware already uploaded and tested on my device,but...How can i edit some conf files if firmware uses squashfs (squashfs as i know is not writable filesystem) ?
2 - is squashfs really readonly? how can i change lan ip of my router if it not writes changes and keeps them in RAM? i think it's readonly but when i change something in router conf usin web gui, device makes new image or maybe  rebuilds it's image and rewrites it. 
3 - Sources of webinterface uses java to edit some conf files, all of them linked  usin this scheme ipcfg.html->ipcfg.cgi, but i can't find those cgi files, maybe httpd uses aliases,so html are equal to cgi or those cgi file compiled into busybox?

sorry for my bad english and hope someone can help me,it's my first try to make custom build of firmware.