One article I read mentioned that DMSA should not be taken with food, as doing so decreases the absorption of DMSA. Some substances are not very stable after being diluted with water. Are you sure DMSA is stable after dilution? Perhaps if it isn't mixed with much water, and it is refrigerated after dilution, it might be stable enough to keep for up to 12 hours?

"I decided to mark the side of a water bottle in 8 separate 50ml increments and I can use that to measure my doses in sips, which saves me having to play with little piles of power from caps."

That would not be a good idea. Perhaps dissolving one capsule in a one ounce medicine bottle full of water, then refrigerating that might be reasonable for keeping it for up to 24 hours. Diluting it in 400 ml of water and keeping it at room temperature might very well cause a major degradition. Do not try this unless you are sure that it would be stable if treated like this.