Lever activated upright spear traps.
Mod in glass spears and use those.
They are crazy sharp, so will cut well, but due to their low other weapon stats, shouldn't cause much in the way of serious trauma. By having the spear trap with just 1-2 spears (test for best results), you can ensure nice blood loss without causing limb loss or bleeding out.
If you have your area set up like this:
W == wall
^ == spear trap
# == wall grate
%> == pump stack that outputs to the right
D == Door
X == floodgate
H == channel/open space
~ == water
+ == smoothed floor
Z + 1:
wDwww
H%>HD
wwwww
Z 0:
wwwwwww
~ww^#Hw
wwwwwww
Z -1:
a holding area and then drainage system. Make sure you use hatches rather than floor grates so none of your blood washes away.
How it works is that you have a pit zone in the same square as the pump stack's water output, and pityour elf in. He lands in the 1x1 square above the spear trap. He can't path anywhere, so will always be here. Once you have triggered your spear that you have sufficient blood in the area for your use, activate your pump. It should wash all the blood down past the elf, through the wall grate, down into your reclamation area. This way your "harvester" dwarves don't get spooked by the elf and produce job cancelation spam. Once all blood has been harvested, repeat.
Note: This design only works if the blood harvesting works like a cross between cleaning and web gathering. I don't know of any other way you could do it.