Hi all,
I was curious about this thing, so sent a mail to Jon. I've just got a reply from him, he said that the Tomato webserver can already serve files not in /www

Basically, just create the dir /var/wwwext (or /tmp/var/wwwext) then put the files there (it's RamFS, so avoid using big files ) and you can access them just opening http://router/ext/filename

Jon also said that only a few certain extensions are supported - txt, htm, js, gif, jpg, png, css. Use .bin if you want to trigger a "save as" download. I renamed a txt file to .asp and it opened it, so possibly can support dynamic pages too.

You can also use symbolic links to files that are outside wwwext (tried personally with a file in jffs), so it should work even with cifs mount (not wasting ram for files ). Not sure if you can symlink directly wwwext, tho.

Hope this can be useful to someone, I'm happy to have something more I can mess up