I accidentally made some burnt maple syrup this spring and planned to add it to a stout, but have not done so yet. I did get to try a Brochet 2 weeks ago and was not a fan. I think it would make a fine addition to a beer with some other flavors, especially some roasted flavors, but by itself, it was sweet and had an odd taste that I didn't care for in excess.
I think it's true that when you cook a honey or maple syrup to a certain point some of the sugars become unfermentable. I don't think there are a lot of resources out there on this particular subject. It might be worth experimenting with 1 gallon batches and writing up an article about it. I bet BYO, Zymurgy, or maybe even HBT could appreciate that kind of thing.