Good discussion. I ignored the muff fumble issue in my first post in order to answer the actual question!
It seems to me that it would be really hard to determine a muff vs. a fumble in this situation. Due to the quickness of the action during a kick, I'm probably going to rule it a muff every time unless it's a really unusual situation. If he drops it anytime between it touching his hands and his placing the ball, it's hard to rule that he actually had possession long enough to make it a fumble - so give him the benefit of the doubt. Just like the rule concerning his knee - he's not down if his knee touches the ground, but what if he shin is also touching the ground, which it probably is!