Impressive. The general approach seems similar to guestfish, is there anything you'd like to do better? Which version of the kernel do you use? I recall Fabrice Bellard didn't use the latest due to a bootloader hack.
Yes, the approach is similar to guestfish, but the JSLinux environment has very different performance characteristics from a standard computer and needs one to be frugal in every dimension: small (4MB) root filesystem, low CPU and IO bandwidth.
We use the same kernel as Fabrice Bellard, with a few additions: jlfs, a filesystem to let Linux view files which are available via the HTML5 FileReader API (multi-file select, drag and drop etc) The actual filesystem browsing is done in userspace on the JSLinux using a modified sleuthkit, and rendered in the browser using the elFinder JS library.
I imagine sysad types would find this a pretty neat way to explore disk image files without having to boot up a VM.