As an American, I've never heard of a long black, but it sounds like a great idea. I normally drink brewed coffee black, but I enjoy an occasional Americano. The only problem is that they're generally served around 400,000° and I can barely hold them and have to wait hours for them to cool down enough to drink. Sometimes I ask for ice or something to cool it quickly. If I just wait for it to cool, it tends to taste rather old and stale by the time I can handle it. Or maybe I just can't taste it because my tongue is crispy from preliminary attempts. But the crema and freshness is always long gone.
I think that must secretly be the real reason for a long black—by adding the espresso to the top of drinkable-temperature water, you still have a chance at the best flavors and aromas. As the Australian coffee roasters Five Senses put it,
Regardless of the crema, the long black tastes best as it cools slightly from boiling to a drinkable temperature. Only then will you be able to perceive some of the finer nuances and sweetness of the coffee.
So, I would shoot to have the cup around the same temperature as good brewed coffee by the time it hits the drinker's lips. According to Coffee Detective,
[American-style brewed] coffee is best served at a temperature between 155ºF and 175ºF (70ºC to 80ºC). Most people prefer it towards the higher end, at about 175ºF.
That means, depending on cup type and ambient temperature, if you draw 200°F hot water from the machine before you start making the espresso and leave it uncovered, it will be roughly the right temperature to serve by the time the shots are ready.