OK a couple of quick clarifications.
.visible is not a command. Its a property. And the preoper syntax for it is _visible (the underscore is a must). So you might try something like
cmpScrollBar._visible = false;
Also, May I suggest that you force your users to wait say 10 or 15 seconds or until say 30% of the text has loaded? That way you wouldn't have to worry about anything except making sure that 30% fits nicely into your text field. Think about it: How fast is someone willing to or capable of reading anyways. By the time he/she reads the 30% of stuff u loaded, some new text would have trickled in, and so on!
That being said, and if you insist on proceeding in the fashion you described earlier, perhaps you can try setting up a listener and onLoadProgress you do some checking to see if there is enough text for a scroll bar, and only then would u reveal it by hiding the movie clip on top of it.
So it would go something like this:
var listener:Object = new Object();
var loading_stream_obj:Object = new Object();//This would
//probably be your File or XML object or whatever can keep
//track of how much has been loaded
listener.onLoadProgress = function()
//calculate the number of lines or find out how much has been loaded
//if enough has loaded
//do something like: myMaskClip._visible = false;
}//end of listener
I am not an expert myself, and I know what I just wrote might be confusing, but I am hoping that at least it might give you some ideas to work on.