Os ringwraiths podem usar um palantir?

Não há provas concretas que sugiram que não pudessem

Os Espectros do Anel eram homens, cujas mortes foram impedidas pela dominação de Sauron. O maior dos Espectros do Anel, o Rei dos Bruxos, era um Rei dos homens e de grande poder. Ele, no entanto, não era do povo de Elros de Númenor. Ainda assim, seu poder ainda era muito maior do que o do jovem Hobbit, Pippin, e apenas pelo uso da pedra é provável que eles pudessem usá-la, mas não tiveram grande sucesso com ela. Também é improvável, já que suas vontades eram inteiramente dominadas por Sauron, ele lhes daria a chance de usá-lo como ele se viu como um candidato maior para seu uso.

Foram os Númenoreanos que foram presenteados com o palantíri pelos Elfos do Reino Abençoado que visitaram Númenor. Aqueles que foram mais bem sucedidos em usar as pedras onde aqueles que eram "usuários legítimos", que eram os "herdeiros de Elendil" ou aqueles que "herdaram a autoridade":

In the case of Denethor, the Steward was strengthened, even against Sauron himself, by the fact that the Stones were far more amenable to legitimate users: most of all to true ‘Heirs of Elendil’ (as Aragorn), but also to one with inherited authority (as Denethor), as compared to Saruman, or Sauron.
_{Unfinished Tales - Part Four, Chapter III: The Palantíri}

Denethor foi capaz de deter a dominação de Sauron por causa de sua linhagem, bem como sua grande força de vontade e força mental, ao contrário de Saruman, que rapidamente caiu em engano:

Saruman fell under the domination of Sauron and desired his victory, or no longer opposed it. Denethor remained steadfast in his rejection of Sauron, but was made to believe that his victory was inevitable, and so fell into despair. The reasons for this difference were no doubt that in the first place Denethor was a man of great strength of will, and maintained the integrity of his personality until the final blow of the (apparently) mortal wound of his only surviving son... And in the second place the Anor-stone was his by right, and nothing but expediency was against his use of it in his grave anxieties.
_ibid.

No entanto, pode-se arriscar o contato com um poder mais strong puramente por sua força de vontade, o que Denethor fez, embora sua linhagem certamente tenha desempenhado isso.

He must have guessed that the Ithil-stone was in evil hands, and risked contact with it, trusting his strength. His trust was not entirely unjustified.
_ibid.

Portanto, parece provável que, embora os Espectros do Anel pudessem muito provavelmente usar a pedra, eles teriam seus pensamentos e enganos vistos por Denethor ou Aragorn, já que eles eram legítimos usuários das pedras, e teriam maior poder por direito.

Finalmente, um grande resumo sobre quem poderia "legitimamente" usar as pedras, a insistência constante de que estas são apenas as pessoas que poderiam usá-las "legalmente / legitimamente" sugere que outras pessoas poderiam usá-las, mas aquelas que certo foram mais bem sucedidos.

These Stones were an inalienable gift to Elendil and his heirs, to whom alone they belonged by right; but this does not mean that they could only be used rightfully by one of these ‘heirs’. They could be used lawfully by anyone authorized by either the ‘heir of Anárion’ or the ‘heir of Isildur’, that is, a lawful King of Gondor or Arnor. Actually they must normally have been used by such deputies. Each Stone had its own warden, one of whose duties was to ‘survey the Stone’ at regular intervals, or when commanded, or in times of need. Other persons also were appointed to visit the Stones, and ministers of the Crown concerned with ‘intelligence’ made regular and special inspections of them, reporting the information so gained to the King and Council, or to the King privately, as the matter demanded. In Gondor latterly, as the office of Steward rose in importance and became hereditary, providing as it were a permanent ‘under-study’ to the King, and an immediate viceroy at need, the command and use of the Stones seems mainly to have been in the hands of the Stewards, and the traditions concerning their nature and use to have been guarded and transmitted in their House. Since the Stewardship had become hereditary from 1998 onwards, 15 so the authority “or again to depute the use, of the Stones, was lawfully transmitted in their line, and belonged therefore fully to Denethor.
_{ibid, note #16}

    

Parece que uma strong vontade é necessária para que um palantír funcione para você. O Nazgûl certamente tem isso.

Aragorn parece sugerir que seu direito de propriedade o ajuda a controlá-lo.

‘Nay, my friends, I am the lawful master of the Stone, and I had both the right and the strength to use it, or so I judged. The right cannot be doubted. The strength was enough – barely.’

_{The Lord of the Rings Book Five, Chapter 2: The Passing of the Grey Company
Page 780 (Single volume 50th Anniversary Edition)}

No entanto, não consigo encontrar nenhuma indicação de que a perda do Nazgûl do que poderíamos chamar de alma os impediria de usar um palantír.