Complete success!
Testing horizontal water flow, I found that when opened to a 7/7 water source a dwarf will be knocked off the target tile in the few seconds it takes for the tile he occupies to reach only 3-4/7.
Here's the design I used. It can be extended to each side for as long a distance as required. In my design invaders will have to cross 20-30 tiles of linked reclining bridges on either side of the plate to ensure that they can't outrun the trap, and also that large numbers of invaders can be drowned at once.
->SSSSSSSSSSS->
->SSSSSSSSSSS->
->SSSSSSSSSSS->
->SSSSGGGSSSS->
WWWWWWWFWWWWWWW
WWWWWWWCWWWWWWW
WWWWWWWPWWWWWWW
RRRRRRR.RRRRRRR
S=Water source (7/7) flowing left to right
W=Wall
R=Retracting Bridge
P=Pressure Plate
C=Channel (to stop the floodgate from being destroyed by invaders)
F=Floodgate
G=wall Grate
.=Empty Space
I found that the anything passing over the pressure plate tile will not remain there for long enough to act as a reliable trigger. Therefore the pressure plate should be linked to a Permanent Effect Pressure Plate System as below to ensure that once something walks over the plate the trap will reliably activate regardles of how long they remain on it.
http://dwarffortresswiki.net/index.php/Pressure_plate#Permanent_Effect_Pressure_Plate_SystemThis device is linked to the floodgate and all retracting bridges. When something crosses the pressure plate the trap activates, drops everything standing on the bridges onto a lower level filled with water and uses the floodgate to knock anything standing on the pressure plate down as well (otherwise the pressure plate would be a 'safe' spot).
For further refinement the lower level is just more retracting bridges, which can be drained of the water they hold to recover wanted items, then retracted to drop unwanted items and corpses into lava. Or kept open to drop invaders straight to the lava. Naturally, this makes a good obsidian factory.
Hopefully, this should kill off all invaders without requiring any actions on my part except to reset the trap. I wanted something as automated, dwarf-safe and foolproof as possible.