scholar:
Why ask me such a question when you are such an expert so you must have an answer. The answer is simple. By the time Neb received his first dream in 606 BCE or thereabouts based on Dan 2:1, Daniel had already been in Babylon for at least 11 years having completed his three years of training becoming unlike yourself, a very wise man. As to whether he was known to Neb during that period of training and thereafter I do not know as I unlike you I am unfamiliar with the protocols of the Babylonian Court system.
Obviously Nebuchadnezzar didn't really know Daniel because Daniel is a fictitious character invented later as a trope for analogues to the Maccabean period. But aside from that, in the context of the story, Daniel was specifically appointed by Nebuchadnezzar as the chief of all things superstitious (Daniel 1:19-21; compare Daniel 5:11), so your attempted excuse for dodging even more flaws in Watch Tower chronology by suggesting that 'maybe Nebuchadnezzar didn't know him at any time' is plainly wrong.