Currently in my PhotoShop (CS6/WIN10/64Bit) I can’t access any Nik plug-ins for files that are typed as grayscale mode.
Although monochrome files don’t need anywhere close to the number of controls that color files do, I find some controls – such as structure – really help when I’m working with scanned monochrome files. A monochrome plugin might look like a slimmed-down combination of Silver Efex and Viveveza, with some pre-configured templates.
There is a small, but enthusiastic and growing, set of analog photographers out there and some of them are using B&W emulsions – often developing the film at home. This includes 120 shooters using vintage and pinhole cameras. They are often new to B&W processing, so a tool set that helps with common problems (negatives too thin, negatives too dense, contrast too high, contrast too low, dealing with grain, scanner limitations, etc.) would be a big help.