There are people who raise rabbits in tractors, but I believe they move them a lot more often than once a day (like, 3 to 4 times per day). Some of the hay you put in a tractor is going to get peed and pooped on, so you'll need to remove it when you put fresh in, or things will get really nasty in a hurry. You may find you are spending in time what you are saving in money, but if you have plenty of that, it works out.
A grass/hay only diet (plus mineral block) may or may not meet all of the rabbit's nutritional needs, and it certainly won't be as nutrient dense as a diet comprised mostly of pellets. You will probably find that it takes you in the neighborhood of 14 weeks to get rabbits to slaughter weight. A doe that is eating only grass or hay can't work as hard as one that is on pellets, so you will need to scale back the number of expected litters per year. Rabbits tend to lose vigor if you breed too close for too long; while you might get away with breeding a pair of full siblings for one generation, I wouldn't take it further than that.
What you are thinking about is doable, but you need to do your research. Check on BYH - I know of at least one person there that has done this.