1. A graduate employee/reseacher union can demand a cap on lab hours regardless of employment status. (Actually it can demand anything regardless of employment status if it's capable of concerted action to put pressure on management). That would also be a trivial an extremely justifiable demand, which would make management sound evil if they try to deny it. A media campaign about the poor lab rats might do the trick.

2. "in the state my grad school is in, graduate students are not employees" <- That's probably wrong, in two ways.

First there's the question of whether you're an employee as opposed to the question of whether you can unionize and bargain collectively in the framework of US labor law, and those are two different things.

Second, the fact that management treats you as non-employees does not mean that that is the legal reality. I mean, NLRB rulings are binding in all states. And regardless of legal issues - at the bottom line it's a matter of organizing yourselves, getting in touch with other organized graduate employees in other universities/states, and educating yourselves collectively (= talks, workshops, rallies, whatever, in which you explain things; print materials; departmental discussion+Q&A sessions; orientation sessions with new graduate employees and so on). The struggle between yourselves and management is first and foremost about your collective self-perception and beliefs.