The root of a createjs app is window. I'm not sure if window has a play() function. So, I think you mean how would you play the main timeline level of something from deeply inside nested movieclips. That could possibly be achieved by looking at document, and finding the item with an ID that is 'canvas', or whatever you have yours named as.
An easier solution could be to put a variable somewhere. At the start of your main timeline you could say:
window.main = this;
then from any depth you could say:
Thank you! It works.
Note that in JS in a browser, "window" is synonymous with the global scope. So you can just do "main = this;" and it will automatically become globally available to the current page (this language feature has made a lot of people very angry and been widely regarded as a bad move). But you should be very, very careful about putting anything in the global scope.
Although I know that main and window.main are the same thing, I would still choose to do window.main. It helps to remind me that it's intentionally a global variable. It's a really easy solution to the problem of getting values from one part of an HTML5 experience to a later, disconnected part.