Actually Stacy, the antibody an rH neg mom can make can pass across the placenta DURING pregnancy. THere does not need to be any mixing of fetal/maternal blood for this to happen. This is why the rhogam shot has saved the lives of many babies. In the past, if a neg mom was sensitized to the rH factor, her subsequent babies lives could be in big trouble. (mixing of fetal maternal blood as you described it is how the INITIAL event can happen, now fetal rH pos blood is in maternal circulation..and she makes the antibody) For the next and subsequent children..the ever present antibody can at will cross the placenta at anytime. That is why they had children die in utero from hemolysis.
The current goal is to keep all women from making the antibody at all, so there next children won't have to deal with the potentially fatal condition. As far as I know, delaying cord cutting won't be a problem, as long as the rhogam shot is given if mom is rH neg and baby is pos.
The only thing I have seen problems with, is IF there is a differing of blood TYPES, and mom is say O (carries both anti A and anti B antibodies`) and baby is say A. If blood is "milked' into the baby from the cord, extra blood that may have not naturally been in the babies system all at one time, can cause increased jaundice as the extra blood cells are broken down. Severe jaundice is not routinely seen though, so I am not implying that.