I don't think it's just one thing:
- The foil reflects radiant heat, meaning that the only remaining way to propogate heat is through conduction and convection.
- The foil is an impermeable barrier which helps to keep the moist air (created by boiling off the moisture in the surface of the bread) near the sandwich, which reduces the rate at which the bread dries out, and we need dry items for them to burn.
It's possible that retention of the moisture within the foil slows the heating of the enclosed system as there's less loss of thermal mass, but I doubt that's a significant factor in most situations.
You might be able to test which of the other two is the more significant factor by running a test with foil that's been perforated such that it's not a good barrier but would still be reflective, and see how that compares.
Well, since you asked for it, with a warning that I've neither cooked nor consumed roast hominy:
If you cover the pot with something that doesn't let steam out, it might slow or prevent the roasting. A stainless steel mesh would let the steam out while also keeping the kernels in. Alternatively, you could use some heavy-duty foil with a lot of small holes poked in it. You could probably also shake it instead of stirring, since the kernels will be contained.
As for preventing popping, I'd guess that dehydrating the kernels first would help. Certainly old, dried-out popcorn is known for failing to pop. And the physics make sense; its basically the water heats, creates intense pressure inside the pericarp, which then eventually ruptures, yielding a BLEV explosion. So if you have less water inside the pericarp, you'll have less exploding.
So I'd try dehydrating them before roasting. Maybe just leaving them out to air-dry (though that'd probably risk spoilage), or a very low oven. Or a food dehydrator.
I have no idea about slightly higher or lower temperatures. Possibly, lower temperatures would result in the water boiling slower, allowing it to leak it without exploding. Or maybe higher temperatures would have only a small portion of the water heated, and then it'd explode out, but only slightly rupturing the kernel (because it'd be a smaller explosion due to less water being involved).
Apparently weakening the hull actually improves popcorn yield, at least according to a patent Google turned up. But I'm not sure how you'd strengthen it instead.
Best Answer
One of foil's main functions is providing a watertight barrier to trap moisture inside the food. This can be especially helpful when slow roasting because you're cooking in a hot, dry environment for an extended period of time. Without foil, your roast would have turned out a lot drier and you'd have lost out on tasty juices. Brisket also has a fair amount of connective tissue (containing collagen) that will melt down into gelatin. Given sufficient moisture, the gelatin dissolves and provides a tongue-coating, lip-smacking richness that's hard to replace, but you don't get the same effect if you dry things out too much.
Foil also provides a barrier to direct heat. In the case of a roast, that reduces browning on the outer surface of the meat. Ordinarily I'd argue that this browning adds flavorful compounds produced by Maillard reactions, but the "beefy" taste you really enjoyed might have been the unadulterated flavor of the meat. If that's how you enjoyed it, just go with that!
Now, as to whether you'd get the same effect with pork belly... you might, but it's a different cut, with loads of connective tissue and very tough muscle that would benefit from some added moisture. You'll also wind up with a lot more fat as it renders. I'd expect pork belly to take longer to cook, and a final searing or browning step would be a really welcome addition to get a good crispy skin. Here's a recipe that came up during a cursory search, which uses foil to seal the belly inside an aluminum pan, so the concept is a sound one. I haven't tried it myself, so I can't speak to the relative merits of this method versus other forms of pork belly preparation.