It is mentioned in yoga vashisht's books which is translated by Rishi Singh Gherwal that he performed mental Raj-suya Vajna in paragraph 54-56 :
Saint Vashisht answered: "King Lavana was not bound by the results of his actions, as he performed them without attachment, and with complete indifference to the outcome. One morning while sitting in solitude, the King fell into a profound reverie and thought of performing mentally the Raj-suya Vajna, as performed by his powerful ancestor Harish Chandra.
He thought of all the accessories to the Sacrifice, and in imagination offered oblations to the gods, and went through all the details of the sacrifice. When he awoke from his reverie, it was night-fall. It is a rule that whoever performs the Royal Sacrifice, must undergo great pain and suffering. So the King of the gods, Indra, sent a celestial messenger to inflict suffering on King Lavana.
The mendicant, who appeared before the King, and put him into a trance in which he suffered cruelly, was none other than Indra's messenger. This I discovered by my ascetic powers. So you will see from this episode, that actions performed even mentally have far-reaching consequences. The story of King Lavana is as follows: A mendicant came to the Court of King Lavana and besought him to witness has powers. On the King consenting, the mendicant waved a peacock feather before the King's face, and immediately the King sank into unconsciousness, while his body writhed fearfully.
When some time later the King recovered consciousness, his minister asked him what he had experienced. The King replied "I felt dizzy when the mendicant waved the fan. Then I saw a horse coming toward me. On the mendicant's suggestion I rode the horse and was carried away to a dense forest, where I fell from the horse and felt very thirsty. While I lay dying of thirst, I saw a young woman pass my way with a pitcher of water. I asked for water, but the girl refused, saying that she was low-born and could not give water to the King unless he married her and promised to live with her folks. In my extremity, I consented. I married the girl, but found it very hard to live with her folks, who were no better than barbarians. I moved up on the hill top and built a house. There I lived with the girl for eight years, and we had two sons. Then there was a severe draught, and as a result, terrible famine. People died by the hundreds and thousands for want of food. I started, with my wife and sons, for some other land. On the way, my wife died from fatigue and exhaustion and my sons cried for food. Unable to see them suffer, I killed myself. Just at this point I recovered consciousness." So said the King, and all present marvelled at the strangeness of the dream. They looked around for the ascetic but he was nowhere to be found.
I have also read about a rishi performing mental oblations to god and was later greeted by god.I don't remember the name.Are there more description about king Lavana and interested in knowing more such stories..references to them.