All devices really only belong in a walled garden, not in the untrusted wilds of the Internet.
For the Kindle you should only need to intercept:
http://spectrum.s3.amazonaws.com/kin...stub-eink.html
The code checks the UUID included in this file, but you can just grab it yourself when you are configuring the process.
You'll need to override the DNS for your local LAN and have something that serves the file.
If your wifi access point allows you to manually enter DNS entries (usually open-source fiirmwares), you are half-way there.
if Calibre Content-Server has a mechanism to serve static-files, that would be the other half.
I use a Raspberry Pi W (the 2W is a better choice now that they are in-stock everywhere). You can follow the online instructions in terms of setting up a 'captive portal' which would go through the minimal hostapd/dnsmasq/apache2 configurations.