So, pressed for time, I really wasn't sure what the dwarf zombies would do. The options were three, as I saw them:
1) They'd stop at the edge of the pit and attempt to thrust their spears across. But that would require they'd know the pit was there and react to it.
2) They'd lose one to the pit, turn around and go back to their room, unable to complete their commands. That seemed to require an awfully sophisticated set of commands for that to work, or rudimentary reasoning on the part of the zombies, which seems counter to how they work in 3E.
3) They'd fall in the pit and reward the players for smart use of terrain.
So I chose #3. I'm still not sure it was the right choice, but it was fun, and that's what matters most.