The usual process of tempering (dark) chocolate involves bringing it up to 45°, then down to 29° and back up to 31°.
If I leave the tempered chocolate in a piping bag for a while and it cools down, would the chocolate still be in temper and set nicely (assuming it was still fluid)?
Without chocolate tempering machines or melting tanks it's hard to keep chocolate at working temperature.
The point of tempering chocolate is heating it before it "breaks" - thus tempered chocolate has that "snap" to it you always want to keep intact. If you've done your tempering (understand melting) correctly there is nothing to worry about. Work the chocolate as long as you can - meaning as long as its pliability caters to your needs. Once that train has passed you need to re-temper the chocolate (here is where, again you can loose the "snapping" properties). Hope that helps :)
It will be in temper as long as it's fluid. But the problem is that it won't be fluid very long. Tempered chocolate sets quickly, which is part of the point, but it makes it a pain to work with. Once it has cooled down enough to be solid, you have to re-melt and re-temper.
The best way, of course, is with a tempering machine, which keeps the chocolate at the perfect temperature. If you don't want to blow a few hundred bucks on a very specialized device, you can try keeping your piping bag in a cooler with a heating pad (on the lowest setting) between uses. Or keep it in a warm-water bath, keeping an eye on the temperature (88°F, plus or minus a very small number).
You may have better luck with couverture chocolate, which is a bit more forgiving, and gives very sharp results. That's real chocolate, with a higher cocoa butter fraction. Or you could use the cheap decorator chocolatesque wafers, which set beautifully and taste like wax.
