The Nyāya and Advaita ātman could be though of as beyond the risk of being harmed by transplanted memories and the like, since it lies beyond "personal" experience, it is the sheer fact of being aware, devoid of any content. Hence, since experience is always, necessarily, intentional (that is: it is experience of something), such an ātman lies beyond experience. It is akin to Kant's Ich-denke (I think).
In this way, if I am not misunderstanding him, Ram-Prasad (in Siderits 2010, forthcoming), describes the Nyāya and Advaita ātman. He also adds the further note that the Nyāya ātman is the substance of which consciousness is a quality, whereas the Advaita one is itself tantamount to consciousness.
But of what "use" is this ātman, which cannot be experienced? Theoretically, even if one could prove its existence (and this can be done, according to Ram-Prasad, through the very fact that one remembers, apart from the memories' contents), one still had to prove its connection with one's "personal" feeling of being an I. One's true explanandum, hence, would not be explained through this unconditioned ātman.