We do have specific guidance on the question of the Palantiri, but as a general rule, the Stewards were able to do anything that the Kings could — although they were definitely not Kings, they wore no crown and sat on a humble seat at the foot the throne.
When the days of the Kings came to an end and Gondor was ruled by the Stewards descended from Húrin, the steward of King Minardil, it was held that all the rights and duties of the Kings were theres "until the Great King returns."
-- "The Tradition of Isildur" - Cirion and Eorl (Unfinished Tales)
It displeased [Boromir] that his father was not king. "How many hundreds of years needs it to make a steward a king, if the king returns not?" he asked. "Few years, maybe, in other places of less royalty," [Denethor] answered. "In Gondor ten thousand years would not suffice."
-- "Of Herbs and Stewed Rabbits" (The Two Towers)
Although in some circumstances, like when the Steward Cirion gave the land that would become Rohan, and the title of King to Eorl these acts were made 'provisionally' and would need to be confirmed when some unknown Heir eventually reclaimed the throne.
However, as the years lengthened and no successor came forth, fewer and fewer people expected that anyone would ever claim the Throne again.
To Eorl I will give in free gift all the great land of Calenardhon from Anduin to Isen. There, if he will, he shall be king, and his heirs after him, and his people shall dwell in freedom while the authority of the Stewards endures, until the Great King returns.37
Footnote 37: This was always said in the days of the Stewards, in any solemn pronouncement, although by the time of Cirion (the twelfth Ruling Steward) it had become a formula that few believed would ever come to pass [author's note]
-- Cirion and Eorl (Unfinished Tales)