Would Dark Harvest work on Nasus, or would it cost too much durability loss?

DH is so busted it's probably worth trying. People also took klepto on him for most of last season, so it's not out of the ordinary for him to go resolve secondary.

Not much of a durability loss anymore with recent rune changes. If you really wanted more durability you could take ravenous hunter in the domination tree but Im not sure if it procs from nasus Qs bonus damage (it should)

Or you could take ultimate hunter and spam your R to stack minions faster.

You wouldn't get too much use out of it until you get to 200~ stacks. That is unless you max E and slowly stack.