All pages are located in directory /img/htdocs. Just log in via ssh and start changing whatever you like. There is only one thing to remember: The contents of /img is copied into RAM when you boot your device. E.g. you will have to copy back the modifed files on each startup.
Have a look at the N2100 NOFLASH module. It replaces some html files to get rid of the .swf stuff (buttons and advertisment). You may use this module as a template.
I did modifications of the web access/pages on my N4100+.
I wanted to have an HTTP access to allow internally to test our web site and an https access to gain access to the usual web UI for N4100+ management.
To do the mods directly on the N4100+ I used the WinSCP tool.
My N4100+ runs the 2.0.04.3 firmware. And the HTTP and HTTPS options are activated!
To achieve that I had to modify the httpd.conf file: it's located in /app/etc/httpd/conf directory.
In the httpd.conf file I did the following: -on the DirectoryIndex line I added index.htm as I use this file as the entry point for our website -on the DocumentRoot line, I replace the /opt/apache/htdocs by the directory where I'll upload all my website files e.g. /raid/WEBSITE -I added <Directory "/raid/WEBSITE"> Options Indexes FollowSymLinks </Directory> after the one for /opt/apache/htdocs
These mods made and saved on the N4100+, I went in the /raid directory to create the WEBSITE directory and upload the files.
I created the WEBSITE directory in the /raid directory as otherwise your directory will be likely deleted after a reboot. I also created WEBSITE from WinSCP but you could also -I suspect- create it from the WebUI interface and make it accessible to the users in your ACL -mine is for test purpose only so didn't want that.
I then uploaded the files of our website in the WEBSITE directory: I then amended the access rights. Note I had to redo some upload as I initially uploaded everything in TEXT transfer mode instead of using BINARY for images: better to know that to avoid rework. Having said that I still have problems with some images but no big deal as it is for test prior production.
After a reboot I could access the website with HTTP and the N4100+ WebUI with HTTPS.
So from there, it would be quite simple to produce your own HTML pages and wherever needed redirect the user to the N4100+ WebUI main page.
This message is kindly accompanied with all the usual & formal warnings/precautions regarding any modification you can do on your units based on the above comments.