Can class D fire extinguishers be used for other fire types than metal fires?
Unfortunately, as far as I can tell, specific class D fire extinguishers aren't even suitable for all types of class D fire, much less other types of fire.

The copper powder variety in particular has limited effectiveness against fires that aren't lithium-based, and the thermoplastic additives in the sodium chloride-based extinguishers may also catch fire.

Dry sodium carbonate-based class D extinguishers should be generally effective but suboptimal against many types of fire, but I'd say in general, even if you have to go a little further, you should go for the proper sort of extinguisher to minimise accidental horrible disasters.