I'm with Dan on this one: hypermiling doesn't lend itself to overheated brakes. I sealed up my plastic wheel covers a couple of months ago and haven't had any trouble at all (though I should say: I don't live in the mountains).
Maybe someone with more experience can talk about the smell of hot brakes vs. the danger of heat-related damage. I suspect if you ever got them hot, you'd start to smell them long before they were in danger of warping/bursting into flames/boiling the brake fluid.
Also one tidbit I remember from a racing course I took: because the brakes were so hot at the end of each lapping session, the instructors always encouraged us to stop the cars in the pits with the clutch (engine off), rather than the brake pedal. Their rationale was that squeezing the calipers that last time when parking the car meant the pads wouldn't retract as much (compared to while driving), and they might create (insulate?) a hot spot on the rotor, leading to warping from different rates of cooling.
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.
A lot warped rotors out there are as a result of over-torquing the lug nuts. Make sure if you take it to a shop or rotate the tires yourself, then torque to the factory specs. ...and by the way, welcome to GS!