This looks cool, though running e.g. arbitrary ruby scripts seems kinda dangerous when it executes on every character instead of only when you ask it to.
You may also like to have a look at https://github.com/akavel/up as an alternative. It now executes the command only after Enter is pressed (though originally I also went with execution on every keypress). By default it runs shell commands/pipelines, but you can change the contents of SHELL env. variable to write raw ruby, awk, or else.