So I rappelled down the side (ran in, catching myself periodically with my climb skill to reduce the falling damage) and found a group of spellcasters and the zombie I'd been hearing through the walls earlier. I managed to shoot them down, and upon exploring the area further, discovered that the hole in the ceiling is the only way in or out! There's no elevator, the hole is in the middle of the room (away from any walls) and I have no ability to levitate.
So my only way out is Alt-F11. I considered waiting until the quest's time was up, and then Alt-F11ing out, under the idea that when I didn't return, they did, in fact, send someone to avenge me (they said they would)... who then finds me and lowers a rope. But I also figured they'd give me credit for the kill, even if I did have to be fished out afterward. Thus, I'm going to Alt-F11 out, declaring that I piled up the coffins and stuff that are in the room so I could reach the hole in the ceiling, and climbed out.
It's amazing how well this game holds up after so many years.