I've never used Jack Daniel's honey whiskey in my mead, but I have used it (and other labels) for curing drinking horns. Just from my exeprience, I'm with wayneb, they flavors are VERY different. For one, it's hard to find someone who uses fermented honey in their mixes and doesn't just drop some honey into their finished spirit. So you get a kind of 'syrupy' mouth feel that I'm not a fan of.
If you're going for raising the ABV, I'd suggest making a 1 gallon trad with the least memorable honey you can find (usually a pale clover for me) and ferment it to dry, then blend. You'd affect the flavor a lot less.
If you really want the whiskey and oak flavor, get some medium toast oak and soak it in a jar with a quality whiskey for a few weeks/couple months. Then put that in your mead. I wouldn't use something Jack, but that's me...
If you absolutely must top off and don't want to use your own mix of honey/water (I usually mix it a few points higher than what I'm at so it has some room to be fermented if the yeast is still active, but won't affect my ABV too much), then go with fatbloke and use vodka or everclear. Something that won't affect your flavor profile too terribly much.
Angry Viking Hedgehog say "Give me mead or I poke ya!"