Technical documentation are often provided in HTML. I do not see any practical reason to prevent access to the web browser while in airplane mode to access local html files. I did not check in that last firmware but I assume that this is still not possible.
A directory should be reserved in the internal storage to place the HTML files. That directory should not be scanned at boot since it may contain images files.
Ideally, the Kobo should also provide a small local web server. The version of busybox in the Kobo Touch is already compiled with httpd (see my page
http://www.chauveau-central.net/pub/KoboTouch/ ) so enabling it should be trivial. If busybox httpd is compiled with CGI support then it becomes possible to implement nice features such as gamebooks (If you do not know what a gamebook is then see
http://www.ffproject.com/crate.htm for an example)
I believe that gamebooks would be perfectly suited for e-readers. It is unfortunate that they cannot be efficiently implemented in standard ebook formats such as epub.