Without looking at the scripting commands.... I would have a room trigger that would send an echo to a room where your executioner mob is located. Exec mob would have his trigger set off on said echo text, teleport to the room or use %at%, and find the player in the room and set him as mhunt target. Have mob teleport back to starting room.
That is, at a glance, how I would approach a goal like that. Will it work exactly as expected? Not sure. If you are still stumped, let me know and I can give it a shot.
If you have a group in the room, the room trigger can toss a variable on the player. The mobs doing the hunt can then search through the room for someone with that variable and target that person.