If you don't mind, your post instantly reminded me of Ubuntu Studio and AV Linux, which I stumbled upon a while back. You might have heard of them. They are very similar to your idea of a DAW OS - they're specialized Ubuntu/Linux flavors built for multimedia professionals. Even more interesting is that AV Linux can be carried around on a pendrive, like you said, and run on top of another OS(Win/OSX/anything) specifically for music production.
These are actually OSes and not DAWs but they are easily the closest to your idea of an ideal DAW. They're just one step short of being a system that is just a DAW. Maybe that'll come true as a distro of Linux based around Ardour.