wow i just solved it.
Problem was that my script was being executed before editor completely loads itself. and i am still unable to figure out why .load() is not working with MCE though .load() work successfuly on other iframes i tested.
So what i did was that i made a click event and .keyup() was attached to iframe body through that function. In my case this scenario was tolerable and feasible but may not be for your case.
so following is a chunk of code that worked perfectly and was showing alerts on every keyup in JOOMLA editor.
$(".myDoButton").on("click",function(){
aaa= $('.myPreviewEditor iframe').contents().find('body').keyup(function(){
alert("hoklo");
});
though this is only a temporary solution.