I think the best bet is michael's first suggestion and that's to make 2:1 syrup and water as needed. You can make more syrup that way and it requires less storage space. Water it down as needed.
Michael,
Great that the Vinegar kills the mold, but what do you do to kill the acetobactor? Do you use distilled white vinegar? Otherwise you are creating a breeding ground for acetobactor which is the bacteria that converts carbohydrates into acetic acid (vinegar).
Keep the chlorine away from teh sugar syrup, it doesn't come out very easily. You "could" try sulfites, but I don't recommend that either. Just make the stronger syrup, and water as needed. I just make it as needed, but then again I am only making syrup for 4 hives right now. I use 1-5lb bag of sugar, and water to consistency. When an operation gets large, then I could see the need to store the surplus syrup that might have been made.