I have already made a few attempts at turning Nikola, a static site and blog generator into a webapp, with varied success. One of the bigger problems was, I think, one of scale. I was thinking of something to replace http://wordpress.com ... something where everyone could host his own blog in very low cost infrastructure.
But then again... why not try to replace all those self-hosted wordpress instances out there? So, here's the webapp plugin!
It's very early stages, no support for pages, only supports markdown, no configuration changes, and you can't even really delete things. But hey, you can post, which is 95% of the usage this will get.
UPDATE: if you are running Nikola from github master, you can install this using the plugins repo: http://plugins.getnikola.com/#webapp and it supports a lot more than on the video above.