Ann, here are my thoughts on it (failed candy maker that I am)
Your pen--technically--will work since the range goes up over 500 degrees. Candy ranges from the 100's (tempering chocolates) up through 300's for various sugar stages.
The problem I see is the "instant" aspect. Understanding how sugar "changes" as the temperature increases (soft ball to hard ball to hard crack, etc) you need to know the temperature of your ingredients at all times. Leaving a thermometer IN the bubbling sugar allows you to watch the mercury approach the temperature you need...and most important, pull it off the heat so it doesn't continue into the next phase.
I don't see how an "instant" read would give you that advance information. It might be "too late" by the time you probe the bubbling sugar.
Some folks know how to read "sugar candy" instinctively and don't need a thermometer. See GayleMO as an example. Then there are people like myself who own 4 different thermometers and still can't make fudge or temper chocolate to save my life.