The grip I find most effective in the snatch is the finger lock. It is opposite of the hook grip (for deadlifting). So, it is thumb over index finger gripping towards the corner (not the middle) of the bell. When I start the drop, I lead with the pinky (I don't flip the bell over my hand). I catch it in the finger lock. Avoiding the usual callous area is impossible, but the bell is primarily in the fingers and not in the palm.
The drop in this fashion will depend on the overhead position. When the bell is locked out overhead, it is on the heel of the hand. I've got some snatch videos on Youtube (can't access it at work) under the account cimes01. Also, Valery Fedorenko's videos are good if you pay attention to how he locks out and how he drops it. Those would be under WorldKettlebellClub.