Well, in my perspective, I wouldn't worry at all about overheating your brakes by putting on the moon hub cap's.
Regarding what it's like to overheat. Their are several things which will happen. First, and one of the most important, is when they start to get hot, you start to loose some of your stopping power. With rotor's, the amount of impact is profoundly less than with drum's and consequently, although they can get pretty hot, you can still stop the car, although it can take a significantly greater effort. Caution, if you try to slow down and you can feel that it's taking more effort, then you need to do something to let the brakes cool down. The best course, if you have a stick is to go down a gear or whatever it takes to get your speed down and keep it down, so you don't need to use the brake's.
Second, when you start to heat the brakes up, they do smell and while it's hard to describe the smell, it basically smells like something is getting smoldering hot and smelly.
I've managed to warp rotor's and drum's, but in both cases, what it came down to was I was going downhill, in a heavily loaded vehicle, with a loaded trailer and with an automatic transmission. Additionally, in going downhill, their were also a lot of very steep, hairpin turn's, such that if you did not keep your speed down and keep it down, you ended up standing on the breaks, on every turn, as you go down the hill. It generally takes a pretty steep descent and a large number of hairpin's, to create a circumstance where thing's start getting hot, as far as brake's, rotor's, drum's and so forth.
The only other thing I can refer you to, is if you find a long, fairly steep grade, which semi's have to come down, if you think of the "unique" smell you encounter, that is hot brake's, rotor's and drum's.
In short, I think that unless you are driving something heavily loaded, down hill's, with a lot of twist's and turns, you are probably not going to need to worry about getting anything to hot.