Mine would nap at the end of tummy time if his big sister didn't come kiss him so much
Belly sleeping helps for naps, I just try to be noisy and check on them a lot.
J is totally "spoiled" since I've had mastitis and had him constantly by my side for almost a week straight. Before that he'd drift off on his own. I think if you start putting them down young they can get into that habit- if their temperament is right.