If I were doing this for a 1000 liter batch I'd be more apt to have some serious instrumentation for accuracy and measurment of SO2 levels than I would for a one gallon batch. Volitalization is a concern over time, but for immediate additions it shouldn't make a big difference to a small home mead or winemaker. Volitalization will be much more rapid in wine, mead, etc than it will be in distilled water. The reason sulfites volitalize so easily is because they bind so easily with tannins, thiamin, polyphenoloxidase, aldehydes and ketones. You don't have that problem with distilled water (yup, I know there's O2 in the headspace of your vessel and that eats up free O2 as well). I'm not advocating storage for use in a couple of days, but when I'm racking my stuff over I take most of the day because I have so many stinkin carboys (glutton for punishment) Hopefully I'll be getting some larger capacity vessels soon so I can cut down on the clutter.
That being said, I generally do not add sulfites or sorbate to my meads, but wines are a different story especially when it comes to browning, oxidation and freshness of flavor and aroma. So to me, it's just simpler to mix up a stock solution on racking day and dose each carboy of wine I rack over.
Hope that helps,
Oskaar