This is not a First Amendment issue It's a Fifth Amendment issue.
James Holmes has every right to keep his mouth shut and not incriminate himself.... If he chooses to talk on his own free will than that is his own problem but he cannot be forced to talk with "truth juice."
Even if they did give it to him nothing he says could be used against him without causing a mistrial. So the whole point would be moot, not to mention open the doors for a massive lawsuit(s) against the state via civil rights violations.