Hi,
I have never used xylazine/ketamine for caesarean section in the dog, but I have on occasions used medetomidine (Domitor -another alpha 2 agonist)/ketamine for very aggressive dogs. I didn't encounter any problems. All foetuses survived.
I have often used xylazine/ketamine in cats, including caesarean sections without any obvious problems. Infact a paper in the journal of veterinary anaesthesia and analgesia 2-3 years ago looking at foetal survival in cats demonstrated the highest survival rate with xylazine/ketamine.