I'm really thinking about the same thing for a couple of weeks now. I love webkit more and love most google products. But the fact that my 100GB of drive space become paying after 2 years just doesn't make me happy.
I was first thinking about editing ChromiumOS to enable OwnCloud instead of Drive as the default file storage, but after some digging that seems extremely hard.
As far as my "research" got:
there are only 2 options if you don't want to reinvent the wheel: ChromiumOS based or FirefoxOS based. Both have their issues and perks.
ChromiumOS has a great desktop, but it's tangled on every level with build in google service support. Getting that out is a big task!
Firefox OS has the following structure: Machine > Gonk/Kernel > Gecko > Gaia.
Gaia is the set of graphical webapps, including the desktop. So the desktop interface is also written in JS. This can make everything just a little slower. I'm not sure what to think off it. As for Gecko, it appears that compositing is not in their. There where efforts made in 2009, but I see no evidence that windowing and compositing works at this stage.
The main reason FirefoxOS and ChromeOS boot so fast, is because they apply patches to the kernel. ChromeOS doesn't even check for hardware during boot. When you want this thing to work on pendrives and all kind of machines that's something you are giving up. You must check for hardware. Nevertheless a sub 15 second boottime on devices with SSDs should be possible imho.
In a few weeks it's FOSDEM (Free and Open Source Developers European Meeting) in Brussels (Belgium). I'm planning to talk to the people of Mozilla about this, and I hope someone can give me some advice.
Hope this helps you (somewhat)