Have minifig hands ever appeared "on their own" in an official set?
Proving a negative is hard, so disclaimer first:

This answer is neither exhaustive, nor definitive.

My line of reasoning starts from the assumption that if there were ever standalone minifigure hands, they would appear in the set inventory in a way that is not explainable based on the number of minifigures. Of course, this assumption is not 100% waterproof, as it does not consider cases when a minifigure torso has appeared without hands AND a pair of standalone hand parts were also used, but I'd accept this risk.

Analyzing the list of the 16123 sets, books and gear catalogued in Bricklink that contain at least one minifigure hand, there are no sets where the number of included minifigure hand parts would be larger than the expected amount.

This number is the sum of all the following:


twice the number of the included minifigures
four times the number of four-handed minifigures (like Garmadon, General Kozu or Rio Durant)
four times the number of monkeys
twice the number of standalone minifigure torsoes (like the masthead of the Black Seas Barracuda)


Minus one for each pirate captain, Ogel, Baron von Barron or any other one-handed figure of course.



Some sets do appear to have more hands still, but that is because the number of hands is skewed by the inclusion of alternative figures, like in the case of Shoot 'N Save (FC Bayern Promo Edition), which allegedly contains 8 yellow hands, but only two minifigures (one of which is the goalie with black hands!), but the soccer player is included in four different variants. A similar thing is going on with Red Bus, where all possible flag-sticker combinations are listed to arrive at the 50 yellow hands for 5 yellow-handed minifigures.

If there are any omissions, I apologize in advance and congratulate the finder :)