[c0ncrete]

i've never really messed with instances, but i assume that you should be able to use the zone's player.pl or an invisible "controller" npc script and select a random spawn location by using one of the quest spawning methods. if the zone used in the instance is also a regularly accessible zone, i don't know off the top of my head how you'd go about making sure the boss didn't spawn anywhere but an instanced zone.

EDIT: i'm guessing the instance version and corresponding version of the script you used would allow you to make sure the boss didn't spawn elsewhere.