I dont think sleeping is sarong for hours is good. firstly this will become a habit, esp you're using auto one, can just let it 'rock' for few hours. I let dear son sleep in sarong when he was about 2-3 weeks old onwars but is only during daytime nap each time less than 2hrs, sometimes around 1hr. And most of the time we will try to carry him out from it when he is aslp.
As for the brain damage, i think as long as you dont 'rock' the sarong vigorously, should be okay.
NB babies usually are more alert and sleep lesser during night time, they will adust their own as they grow older. =)