Meh. Totally forgot about Ubuntu Phone for ARM.
You'd have to look into compiling Node Webkit yourself for this, I guess.
Since Node Webkit's Github account lacks any concrete instructions on how to do this, you might want to take a look at Chromium for Linux ARM build instructions (Node Webkit and Chromium are, essentially, both using the same build process).
code.google.com/p/chromium/wiki/LinuxChromiumArm