You can download the ChromiumOS source code, build it, and install it on any available piece of hardware. (Though the hardware can't be too lightweight; I tried this on an Atom-based eeepc and it's just too slow. A virtual machine on a modern desktop works fine, though, if you just want to see what ChromeOS is like.)
You can also install your own build on an actual Chromebook, of course, but you lose automatic updates and the secure boot.
You might also consider porting the features you desire to a NaCl app, or contributing to NaCl to add things you need. (Emacs is on my todo list, though not in the near future.)