From Coffeekup and JAML's source, (while working on question), we can see a way to hack ruby's instance eval into Javascript (JAML author explains more). It involves decompiling the function, and evaluating it around a with block.
The question is: is this supported all around browsers/js runtimes? I know it works on firefox, opera and chrome, but toString on function is not consistent among platforms, hence the question.