Great news: QtScript is there and fully usable, so this might be a great way to develop plugins. I wrote a plugin that exposes QtScript over the network, so you can just telnet to your Kobo and mess with stuff. This allows access to pretty much everything exposed by Qt introspection.
Source (and some documentation) is at
https://github.com/ah-/koboplugins, and attached is a binary version if you don't have the full development environment set up yet. Copy it to /usr/local/Kobo, reboot and open the library once to load the plugin. If you have any issues with it, just delete libqtscript.so and you should be back to normal.
Example use: Telnet to your Kobo (port 1337) and execute "aw.findChildren("label")[0].text = "QtScript""
This changes the text of the "START" menu to "QtScript". Without compiling anything and interactive.