When my sister gave birth to my nephew, she wasn't producing enough milk for him. She had to supplement his feedings with formula. He was still hungry, however, so my Mother recommended feeding him rice cereal at a week old. By the time he was two weeks old, my sister's milk supply dried up, and he was completely formula fed from then on (as well as given rice cereal). He did very well, and gained weight appropriately. He will be five in June, and he's close to 60ibs and quite tall for his age, so he definitely didn't fail to thrive.
Ironically, when my sister had my niece, she did produce enough milk, and was able to nurse her. However, the baby's PKU was diagnosed when she was six days old, so she was taken off breast milk and placed on a special formula that's given to PKU babies.
Now, back to topic...I don't think I would allow someone else to breast feed my child regardless of the benefits of nursing. I would do what my sister did, and supplement with formula if I didn't have enough milk for the baby.