file:/// works but it shows the HTML source code instead of actually rendering the HTML.
For the German version of the Story HD Wifi I made a "Web Portal" mod that loads a local HTML file which displays a list of links you can choose from (including a link to the book shop so you can still use that if you like). For this mod I'm actually spawning a small web server on the Story HD so it can load
http://localhost/webportal.html, because I couldn't get the browser to render HTML from a file:/// URL.