Warfarin apparently tastes like vanilla. So, I can see where it could be in food without raising the victim's suspicion. But, I would think that it would have to be in large quantities, and I'm not sure it could act that quickly.
DMSO will carry stuff throughout the body, but that stuff tastes like oysters. Immediately you get this taste in your mouth.
But if they served oysters.....
I would think Coumadin would take awhile longer to act. I once worked on a case for a lawyer where a prescription for coumadin was filled for an elderly man at a much greater dose then he was supposed to receive. Even so, it was several days before the effects became apparent and then it was more like heavy blood pooling under the skin. No bleeding right off from the nose or ears.