Personally I can only thing of trouble that could come from using a sub as a surrogate. An impartial 3rd party is probably the best solution.
Being in complete control of them can also cause problems even if it was a sub, again bad idea in my mind. Regulating everything could cause stress which will harm the baby. Having never researched it I can't know for sure. But knowing the lawsuit happy world we live in, I would think the angency would be very very selective in their choosing surrogates. Obviously there are doctors appointments to monitor and test to make sure they are taking care of themselves properly. I'm fairly certain they would have to follow the diets the doctors give or again lawsuit central possibly from the angency if they went against the contract I assume they would have to sign.