Most all maples can be tapped. As can walnuts.
There was a trial done some years back... results published in the Northern Nut Growers Assn. yearly proceedings... Boxelder/Manitoba Maple syrup actually got better flavor ratings than sugar maple.
Necessary temperature/weather conditions may be a bigger stumbling block for folks trying to tap in more Southern locations than tree species.
I make hickory syrup, but I make mine from the nutshells left over after I crack and pick out my select shagbark & shellbark hickory nuts - cleaner than pulling off strips of bark... no lichens, poison ivy, bird poop to deal with. And, weather doesn't come into play.
I just boil nutshell fragments(there will be some pieces of kernel in there, too) - and, anymore, I just cook 'em all day on High in the crockpot, then strain the 'liquor' through an old t-shirt, add 1-1.5 cups sugar per cup of 'liquor', cook that down to desired thickness and bottle. If I want it a little darker, I'll throw in a few clean nut husks when I'm boiling the nutshells. I usually boil a few intact nuts when I'm cooking down the syrup, and put in one nut per jar.
Don't have any good shagbark or shellbarks? I didn't have any back home in AL.
You can make this with mockernut hickory nuts... I'd just crunch 'em with a pair of vise-grip pliers and toss 'em into the pot, kernel and all.