I just tried that earlier today, but I still couldn't plant the bomb. I suppose it's conceivable that Essence of the Nightbringer wore-off by the time I got Kais right to the spot, though.
For those people who have trouble with the bomb marker not activated at the bomb site after killing all necrons. I've ran into this problem a couple of times with different races (chaos, SM, orkies), and seems that you need to plant the bomb before you kill the necron commander. If you just sit on your butt and wait until the nightbringer dissapears, kill NL and clean all the remaining tin boyz off then bomb doesn't activate. Looks like you need to run your commander past the NB in order to plant the bomb.