Found this, in case it helps anyone:
When the editor suggested this topic (the elusive hickory syrup) I recalled having seen some mention of the syrup while browsing in Dave's Gardens forums. In short order, I relocated the posts that I remembered. A DG member had said that hickory syrup was easily made using cracked shells of various hickory nuts, as well as by boiling shed hickory bark. He was kind enough to email me a copy of his writing on the subject of hickory syrup making.
Self-described hickory "nut" Dr. Lucky Pittman tells about his experience with homemade hickory syrup in a paper submitted to the Northern Nut Growers Association newsletter. With his permission, I present the following instructions for making your own hickory syrup from bark or nutshells.
a large pot full of cracked shell and husk, or cracked whole nuts from shagbark or mockernut hickory, or of exfoliating bark scraps collected from shagbark, shellbark or pignut hickory trees.
Wash and drain the nuts, nutshells or bark pieces to remove loose dirt. Put the bark or shell into a large pot and cover with water. Boil the mixture all day. (Makes the house smell good!) Strain out the solids and measure the liquid. Return the now brown, aromatic hickory "liquor" to the pot and add sugar in a proportion of one and a half times the amount of sugar as you have of liquid, for example four cups of liquid needs six cups of sugar. Boil this for thirty minutes. Pour the syrup into canning jars and seal them. (Not specified, but I would suggest you may want to store in the refrigerator) You may adjust the amount of sugar a bit but too much sugar will simply crystallize in the jar.
That sounds simple enough. I will give certainly give the Hickoryworks folks due credit; I'm sure they've been diligent in standardizing their recipe to turn out a consistently high-quality product. A video on their home page will give you a little more insight into the technical aspects, such as Brix testing, of their process. Between hints from Hickoryworks, and the experience shared by Dr. Pittman, I think we have the makings of some fun experiments in hickory home brewing to warm up the rest of autumn. Good luck, and let me know how it turns out. I'm off to identify some local hickories.
There are some other hits on the Net, if you search "Hickory nut/bark syrup".