I haven't had to assist feed neonate ball pythons but I have with BRB's. The longest was just over a year; she was from a weak premature litter where only two survived and there were lots of stills and slugs. The larger of the two was assist fed a few times, the smaller had absorbed very little yolk and was about half the size of a healthy neonate.
They were actually too small for even a mouse pinky so I started them on a hind leg cut from a f/t mouse hopper. That was fairly easy to feed since I would bend the leg at the hock joint and insert that into the snake's mouth. The baby couldn't spit it out easily since the leg would open up and get stuck in the jaws behind the teeth. Once the neonates got a little bigger I would chain a whole prey feeder such as a mouse pinky, fuzzy, etc. behind the hind leg. With practice I could get the process done in under five minutes.