Well crap. I came back and bought a Nook book with my settlement credit, figuring I'd de-DRM it like I always have in the past, only to find that neither Nook for Mac nor Nook Study will run on my Sierra machine. Sic transit gloria Nookie.
The Nook color tablet was my first tablet and my first e-reader. With Nook books now effectively imprisoned, I don't have any motivation to buy from them again.
Dammit B&N.