I figure that at some point someone has devised a demosaic algorithm that is optimal for converting a Bayer DNG to a B&W photograph.

Well "optimal" is in the eye of the beholder.

Your idea of an optimal conversion may not be my idea of an optimal conversion. So on this notion the idea of an optimal conversion flounders.

On a fundamental level, demoisaicing algorithms are concerned with generating RGB images from RAW data.

And B&W image processing, even in the days of film, involved color filtering.

With film you actually did use a physical filter when you shot in order to get different contrast effects for different colors. In digital, normally, you'd work from a color image and apply a filter of choice (or any other processing) to get the effect you want.

The point is that B&W processing is almost entirely something you do now from a color base image (implicit even if you shoot RAW).

Even if you attempt to do this from RAW, you must construct an implicit full color image in order to apply a color filter-type of effect (something I'd regard as pretty much essential for serious B&W work). A color filter, by definition, requires a color source image.

The only alternative is rarely done these days with digital - shooting with a physical color filter.

But what is that algorithm called, and what image processing software offers it?

There is no single algorithm that would be universally considered optimal, but I can't say I've heard of an algorithm specifically designed to facilitate only B&W work. As I've explained, it's a self defeating idea to not work from a base color image these days.