Update: The new file it's trying to access appears to be
http://www.kobo.com/buildinfo.xml
I was able to set up a virtual host in my raspberry pi webserver to fake
www.kobo.com and copied that file there, and make progress; now the kobo, when attempting to connect, will successfully connect, but then throws up the "this appears to be a public hotspot, would you like to continue" dialog. The dialog simply displays the text from that xml file. I suspect I may need to create further fakery.
I'm unsure what to do next, but hey, it's progress.