Shouldn't work... butchering sentients is broken as far as I know, and adding pet tags wont let you eat or use the meat, they will still act as if its junk...
What I would suggest is creating a fake creature.... call it CREATURE:SENTIENT make it not spawn naturally. add in each caste of every race you are going to want to kill (basically CASTE:DWARF_MALE and CASTE:DWARF_FEMALE, etc etc etc), select all the castes for each race individually, add all the creature information for the entire race then, except tags that involve learning and can speak, basically they will be normal creatures that look like dwarfs or elfs or whatever. after that add in caste information for each caste individually. repeat for each race.
now comes the fun part. DFHack a script that identifies "unit targets" in range of the butcher shop, verifies they are NOT your civ/group, verifies their race and caste, then uses create-unit to create -race sentient -caste {prev_race}_{prev_caste}, and use create-unit's domesticate function on the new unit. same script should teleport the old units to 0,0,0 and set flag scuttle on them.
After that create a reaction at the butcher shop (something like "prepare captives for butchering") and make a reaction trigger in onLoad.init to target this reaction. the reaction should have 0 reagents and products, just give it a skill if you want it.
after all of that, if you did it all right and didn't mess up on creating the script and the sentient unit... you should be able to build cages around the butcher shop, hit the reaction and fake sentients will appear with the right race and caste names... then you simply tag them for butchering and you got elf hippie skulls!
the only reason i went this deep into the discussion... I've already done a similar thing before, and I know scripts that do each step already that would be able to perform this.... but otherwise your not going to be able to do it... if you want to really have a blast with it... you could have the created unit be set to scuttle also so they just appear as dead bodies around the butcher shop ready to be butchered. Otherwise... without DFHack you wont be able to butcher "sentient" creatures and even with DFHack... I doubt you would be able to transform a dead unit body into another creature...
the game is just broken on sentient butchering.