I'm not a lawyer (but I do watch Legal Eagle!). I think the person would need to be declared "mentally incompetent" first. The laws for that vary from state to state. My guess is that yes it's possible, but easier said than done.

If one has the mental fortitude to plead, then possibly either:

1. The court would reject it because the judge would be confident the person was lying. Or,

2. The person would be sent for psychological evaluation, and if the person was deemed unfit to stand trial, they would continue to remain in a psychiatric hospital until such time that they are not deemed a threat to themself or to others.

I'm not a lawyer, but I think psychosis is a medical diagnosis, whereas lawyers would be concerned with legal insanity. The latter can draw from the medical diagnosis, but psychosis doesn't guarantee that legal insanity is present, and one can prove legal insanity without a psychosis diagnosis. Certainly, if a doctor will confirm that the client has psychosis, then it's easier for the lawyer to prove legal insanity.

The NIH provides this helpful definition:

What is psychosis? Psychosis refers to a collection of symptoms that affect the mind, where there has been some loss of contact with reality. During an episode of psychosis, a person’s thoughts and perceptions are disrupted and they may have difficulty recognizing what is real and what is not.

Essentially, psychosis sufferers are "disconnected" from this reality, and don't see everything we do, and see things that no one else does. But what legal insanity tries to capture is whether a person was able to understand right from wrong.

Example 1: a person suffering a psychotic episode attests that he saw demons about to attack his spouse on the driveway. He then produces an AK-105 and fires at the non-existent demons to protect his spouse, but the bullets go into the air and into the stucco walls of his neighbor's house. No one is injured.

Here, a doctor could conclude psychosis, but a jury might not find legal insanity. The person's reasoning was correct (ie using deadly force to protect someone in danger) but his perception of the non-existent threat was faulty. He clearly knew right from wrong. From the law's perspective, this becomes a case of mistaken facts, like burglars breaking into the wrong house, or a petty thief stealing something more valuable than intended. Many courts would find the defendant guilty and remove the guns from his possession, but may issue a shorter custodial (ie prison) sentence at a state psychiatric hospital, sufficient to bring the psychosis under control, then serve the remaining term.

Example 2: a person suffering a psychotic episode attests that he saw demons about to attack his spouse on the driveway. Because the demons are monstrous in size, he believes his spouse will be killed no matter what. He believes that the spouse's infidelity has wrought these demons upon her, and that they too are punishing him by killing his spouse. Thus, he produces an AK-105 and shoots his spouse, so that the demons will not have the satisfaction of killing her, and he will have avenged her by killing her. The spray of bullets kill the spouse, go into the air, and also into the stucco walls of his neighbor's house. No one else is injured. Later investigation shows the spouse was never unfaithful.

Here, both psychosis and legal insanity probably exist. Seeing demons -- of any size -- is evidence of a psychotic episode. The contorted logic of avenging someone by killing them is evidence of an incapacity to see what is right from wrong. This is not just the facts and perceptions being clouded, but a wholesale loss of sense and judgement. From the law's perspective, this person is unable to take care of themselves, let alone function amongst society. Many courts would find the defendant not-guilty by legal insanity and remove the guns from his possession, but will issue an indefinite order for committal to an institution until fully treated for psychosis. But this is often longer than any prison sentence for the crime. The other option is that some courts suspend the trial, send the defendant to the institution until fully treated, then resume the trial, if possible; a treated defendant might still be unable to aid in their legal defense, for example. Again, the defendant could end up being locked up somewhere for a very long time, even without being convicted.

All the above depends on jurisdiction, with some allowing an easier pleading of insanity, while others less so. States like California might also prohibit certain related defenses, such as diminished capacity.

So can a lawyer plead insanity? Sure, but it's a long shot, with one reference saying that less than 1% of defendants succeed in their claim. And if it is successful, the client may end up with more time than in prison.

I would assume that depends on the jurisdiction.