Couldn't the Chimera Ant King be trapped in Knov's Nen dimension?
The plan required the Hunters to somehow force the King to move into Knov's Nen rooms, which would be almost impossible. Knov creating a Nen portal close enough to the King and pulling him into the room is ruled out. As you mentioned, just coming into contact with Royal Guard Pouf's En caused him to suffer a mental breakdown.

The next option is for the other Hunters to move the King into Knov's rooms. The Hunters could not realistically hope to achieve that given the gap in power between Chairman Issac Netero and the King.1 The Hunters were aware of this gap before entering the battle, so they wouldn't consider it. 

The plan was also risky because the extent and nature of the King's powers are a big unknown. Even if they succeeded, what if the King's Nen could destroy Knov's Nen once he is inside the room, or if it could "hijack" Knov's Nen, analyze how it works and obtain the Master Key? (and so on) The Nen abilities in Hunterverse vary from compound interest to teleporting gorillas, so the King having some troublesome Nen abilities could not be ruled out. 



1 The anime confirmed this gap of power rather beautifully later when one of Netero's strongest attacks scores a direct hit on the King, who was sitting on the ground, but the King just wipes some blood off his cheek.