Well, it doesn't look promising with the retraining of lv 100 pokes. I didn't do a full EV training, but I did try to add 70 attack EVs onto a lv 100 Spiritomb. First I made sure it had no EVs with EV reducing berries. I tried to add one HP EV by attacking a bidoof. I gave it 7 proteins, then checked it's stats for changes. There were none. I tried putting it into the PC box to see if those changed and found they did not. Then I tried removing the single HP EV (which probably did not exist) and also put the poke into the box and took it out again, but again, there were no changes in its stats, so I think the box trick doesn't work in Platinum.