I would definitely not do the risotto, very hard to get and hold the right texture in those kinds of quantities. The basmati rice will work fine in a large pot. Cook it by the absorption method (bring to a boil with 1 3/4 cups of water per cup of rice, reduce to a simmer, cook covered, without stirring, until all of the water is absorbed).
The basic differences between white and brown rice is explained below (as written here):
An important first question to ask about all rice-and for that matter, most foods-is how much it has been processed. In the case of rice, processing usually involves milling and polishing. The outermost layer of rice, called the hull, is removed to make brown rice. Brown rice is rice with the whole kernel intact and the kernel is still surrounded by all layers of bran.
To produce white rice, the bran layers of the rice have to be milled off. Most of the rice germ is also removed during this abrasive grinding process. At this point in the process the rice is called milled, unpolished white rice. Finally, a wire brush machine is used to remove the aleurone layer that remains on the rice. This step is called polishing. As polishing is not an all-or-nothing process, semi-polished rice may still contain parts of its aleurone layer.
Additional information about brown rice (here):
[Brown rice] has a mild nutty flavor, and is chewier and more nutritious than white rice, but goes rancid more quickly because the germ—which is removed to make white rice—contains fats that can spoil. Any rice, including long-grain, short-grain, or sticky rice, may be eaten as brown rice.
So, because they are processed differently and have different parts of the grain intact or removed, your method of cooking is going to vary.
I have used this method of cooking brown basmati rice with pretty good success. It includes washing the rice a number of times prior to cooking and adding salt. This method advises adding oil after the cooking process, but I typically add it into my rice while it cooks, or even sauteeing the grains in oil then adding water to the pot to cook.
It is also suggested that leaving your brown rice to soak for 15-20 minutes (and sometimes even longer) can be beneficial as well.
All of that aside: if you're looking to stick with a flavor and texture similar to that of white rice then brown rice may not be your thing. I know that as a jasmine rice eater I do not particularly care for the texture of brown (even brown jasmine). As mentioned above it does have a "nutty" flavor and is "chewier" regardless of how well/properly it is prepared. It changes the flavor and feel of every dish I serve it with. Just a heads up.
Best Answer
Slow cooker
If you have a slow cooker as Catija suggested, you can totally do exactly what you want. Search for slow cooker rice pilaf if you need a starting point. I know you don't want new appliances, but slow cookers are a bit more general purpose, so maybe you have one or would be okay buying one, and future readers certainly might.
Oven
I'm also pretty sure the oven will work, but it might require a little experimentation to get the timing right. There are again a lot of recipes for rice pilaf in the oven, but they normally start with boiling water then take perhaps 45-60 minutes for white rice or 60-75 minutes for brown rice, at 350-375F, so they'd be very overdone after three hours. But if you reduce the temperature, you can extend the cooking time and make it more tolerant of cooking past done. A slow cooker isn't really very different from a tightly covered dish in an oven at a low temperature.
So, if I had to hazard a guess, I'd take a normal oven rice recipe (with the water boiling before starting), and reduce the temperature to 250F, then see how long it takes to be done and whether it's okay after three hours. It'd really be best to do a trial run sometime when you can actually check on it, though.
If that doesn't extend the time enough, you can probably stretch it even more by starting from non-boiling water, but I unfortunately don't have a good sense of what combination of water and oven temperature would accomplish a three-hour cooking time.
Reheating
If you don't want to experiment, your fallback of reheating can work too. The key in my experience is to add a bit of water then cover when reheating, so that it steams and moistens again, rather than getting dry and hard.
For one to two servings I just drizzle a little water in, maybe a couple teaspoons, before popping in the microwave. For a larger quantity like your situation, probably a bit more than that - the idea is that it mostly but not entirely boils away by the time it's all reheated, so that the rice isn't wet, but has been surrounded by steam the whole time. You can always check on it in the middle and see if it's nice and steamy, and add a bit more if it's starting to seem dry.