I have a birch sap Imperial Stout in the carboy which I used the sap partially in the mash and completely for the sparge water. I would have done a 100% sap mash, but with all the roasted grains and the low pH of the sap itself, the mash would have been just far too acidic.
I also have a Maple Imperial Ale bottle conditioning which used absolutely no "water", just the sap. The gravity of the maple sap was about 3 times more than the birch sap, and I feel the character is definitely there in the beer. I'm really happy with it. The beer was even a darker amber than beersmith predicted, maybe an extra 3-4 SRM, presumably from the sap.
I would highly recommend using sap in the wort production if able, but it is season dependent of course, and probably passed for everyone now. Otherwise, adding the syrup after fermentation would provide very similar flavors, if not even more pronounced. One downside of using the sap/syrup is the high manganese content, which I believe can accelerate staling. I wouldn't worry about it though, especially in a strong dark beer. I did not notice any oxidation in my last bottle of Birch Sap Baltic Porter that was just over a year old. I would just be extra careful not to introduce any extra oxygen after fermentation, and bottle condition if you would like to age it out a bit.
Go for it, let us know how it turns out!