That's some really good software architecture! Here is my experience starting out with this tech:
I checked out the samples and I was bewildered at first, but then I found a hello world example for Adobe HTML Extensions: A Short Guide to HTML5 Extensions | Adobe Developer Connection. I noticed there is a change in the CEP 5 extension storage location. In the previous version it was %APPDATA%\Adobe\CEPServiceManager4\extensions but now it is %APPDATA%\Adobe\CEP\extensions.
I setup the Eclipse add-on called Extension Builder, but I found that creating a new project was much easier to do manually. I made a batch script that copies my project files from the network to the local extensions folder and then runs Premiere. I use Notepad++ to edit and Chrome to debug. It seems to be working. :-)
I recommend avoiding Eclipse and Extension Builder entirely; you can also develop directly in PPro's /extensions directory, to avoid any copy step. notepad and Chrome are the way to go.