zdraveite na vsichki, mnogo mi e priqtno da si pisha s vas :)

Can the durock be finished with a thin layer of something so it's flat and smooth?

Tile, brick, stone.

Or you can use DensArmour Plus, mould resistant drywall with a fibreglass mat facing.

But aside from a bathroom, it's unnecessary. You can control the basement humidity with damp proofing and mechanical dehumidification.

Damp-proofing can always fail. I want to finish the durock in some way that it will be unlikely to mold even if there is humidity there.
Tile and stone are not flat and smooth. I was thinking about something like plaster of paris or spackling paste and wondering what would be best. Like when you repair drywall with a putty knife and after it's dry, make it flat and smooth with sandpaper. Only I would apply it to durock.
I would do this just to the bottom 2 feet of the wall, so I wouldn't have a huge area to work on. I might apply tile to the bottom two feet of the wall, but what I was asking about are ways of finishing the bottom two feet so they are flat and smooth.

what I was asking about are ways of finishing the bottom two feet so they are flat and smooth.

Ceramic tile is a lot smoother and flatter than plaster.

But, yes, a skim coat of plaster can be applied to concrete board. It's advisable to use a bonding agent, such asPlaster Weld, first. You might need several coats for a smooth surface.

However, if your idea is that there will be so much moisture and humidity in your basement that nothing but concrete or concrete board can withstand it, then your basement should be properly damp proofed/ waterproofed and dehumidified. BTW, mould will grow on even concrete and Durock.

Fix the humidity problem. You're trying to treat the symptom rather than curing the disease.

"Ceramic tile is a lot smoother and flatter than plaster."

Must be the new super size 'cut to size' tile with no grout lines.