I've been stewing on this too. I have an idea, but it may be somewhat space consuming. Murder Life-span re-assignment room is an arbitrary size, not less than 1x4 open floor space. Floodgate closes door (perhaps behind a regular door that can stay locked to prevent unplanned operation), and anther floodgate permits water from closed system cistern, central water supply, river, etc. One floor square is a hatch-cover for drainage, linked to the reset condition. Drain may encompass floor grate on lower Z-level to facilitate straining. Another floor section is a pressure plate to activate the system. Third floor square is bait assignment device. This could be a bed, do nothing switch, etc. Alternately, the switch could be the activation condition, if you want them to pull thier own trigger. The fourth floor square is a "latch". It is activated by 2/7 water. It is linked to the reset circuit, and does not allow the reset circuit to signal again until water is nearly gone from the room.
On the Z-level above, one tile is channeled, and floor grate installed. This allows for over-flow into the timing circuit. The timing circuit is a closed cistern, on the other side of one wall of the main room, with a linked hatch-cover for draining, as the main room. The volume of the cistern is the timer. Dependent upon the length of time it will take an victim assignee to drown complete their assignment in the chamber, the cistern has to be of sufficient volume to fill with the overflow of the main room, before overflowing, itself, into the reset circuit.
Beside the timing circuit is a spillway with a pressure plate. The pressure plate is set to trigger at 1/7 water. This will only activate once the timing circuit fills and overflows. This spillway drains immediately.
Once the system is activated, the flood gate to the entry passage is closed, the floodgate to the water source is opened, and the room fills with water. Once the volume of the room is full, water flows out of the floor grate on the Z-level above, spilling into the timing circuit.
Once the timing circuit fills, it spills over onto the reset circuit. The reset circuit changes the state of the two floodgates as well as the two hatch-covers. This circuit only fires once, until the state of the latch in the main chamber resets it.
Where the water goes out of the system is another matter. It could go into the cavern layer. It could go back to a central water supply, out the side of the mountain if you're installing this several Z-levels up, go to a pump house with a pump stack set to R, returning it to a closed loop water tank for the system, etc.
It may be adviseable, also, to have a door into the section of drain that has the floor grate under the main room drain. It would act as a cleanout. I'd say keep it locked most of the time, to avoid socktrample after normal operations.
Admittedly, I'm only so-so with interlinking pressure plates and mechanisms for interdependent reset conditions, but I think the concept is sound.
What do you think?