I have been playing Captive Minecraft and set up an iron golem farm, due to the extremely limited nature of available resources. In shoving my artificial village into a corner of my available space (a 31x31 area) the "iron golem spawn zone" extends outside the world boundary some distance. Attached screenshot is after he wandered back closer while I was moving to get a closer picture, as if his AI pathing was only allowed to path to locations inside the boundary, but when I first noticed him he was a good six blocks farther out. Ironically, villagers will attempt to path outside the boundary...

While no other mobs spawn outside the world boundary (even zombie sieges spawn inside my farm area rather than the very nearby, dark, but outside the boundary locations).