I sometimes like to donk the river when the K comes and I've represented AK, KQ hands, but not against unknowns. There are a few factors that I like to know first.

1. Is villian a thinking player that has put me on a hand range?
2. Is villian capable of laying down on the river?
3. What is my table image to the villian?
4. What is villian's hand range?

With an unknown, it's just too hard to know these things. I just check/fold until I know a bit more about the player before using a donk bluff on the river.