I've been working with keyboards for quite a time. I would say that the easiness of this depends on what your target text input view is. If is a UITextView then we are fine, if is UITextField you might have some problems because you don't have access to current cursor text position like un UITextView.
(You might check UITextView and UITextViewDelegate methods)
If you just want to set a string and don't mind current cursor text position, then you don't need a keyboard. (I think this is the case of Tipulator)
BTW: I just saw Tipulator in youtube and there is no necessity of a keyboard for doing that.