OK, I have a way around this, which involves patching libnickel.so. I am extremely grateful to Glyn from
Oxfork Hack Space for taking care of the actual reverse-engineering work while I provided a fair share of unhelpful suggestions and misleading background info.
To patch your Kobo, first install firmware 2.5.2 from
here, get a shell running, etc. Then retrieve the file /usr/local/Kobo/libnickel.so.1.0.0 from your device to your computer (which I assume is running a *nix system). Check that its SHA1 sum is 4c3d7d8cdce4927cbffbde8d3d4c6b7bd35de5c1. Now, install the bsdiff and bspatch tools, download the
libnickel.so.diff file, and run bspatch libnickel.so.1.0.0 libnickel.so.1.0.0.patched libnickel.so.diff. Check that the resulting file has SHA1 sum 4c96ac498e31277236645c2d466db7af9b19f7c5. Replace the original libnickel.so.1.0.0 on your device by the patched file. (Of course, keep the original as a backup and restore it in case in turns out this patch has dire unforseen consequences.) Restart your Kobo.
Now, you should be able to open the browser in Settings without being prompted about connecting to a Wifi network, and you should be able to load URLs such as "http://localhost/".
I should insist that this will
not work on any different version of libnickel.so, so it will probably fail on the next versions of the firmware. I plan to document this hack a bit more in a blogpost so that adventurous people can try to apply it to different versions.
Edit: the blogpost is online:
http://a3nm.net/blog/kobo_glo_hacking.html