I love the spirit of this, and I've been waiting for someone to make it for years, if I didn't get to it first!
If anyone wants to make a similar web app, I maintain an open source library whose sole purpose is putting JavaScript drawing programs on web pages with custom tools.
Here's a simple web app I made with it that saves to local storage and uploads to Dropbox and Imgur: http://drawplz.com
(The text tool is slightly broken right now; the selection box drag corners are invisible.)