I'm talking about the charging circuitry in the Mac (controlled by the SMC). Since the battery pack needs a certain voltage to charge, it would require a more complex voltage regulator/converter. I'd be surprised if it could charge at both 5V and the higher input voltage.
BTW, this also highlights an issue with USB-C: Not all USB-C chargers need to support all possible voltages (the only one that is mandatory is 5V). So it might very well happen that a USB-C charger from one vendor cannot charge another vendor's device.
Macbook 12 has a smaller battery than iPad 3 or 4 (39 vs 43 watt/hours).
So iPad charger will do well on new Macbook.
Unfortunately, Rigby makes some good points here. He's saying USB 3.1 is limited to 3A current, meaning that the MB charger has to be running 9.5-9.8V in order to be 29W. The iPad and iPhone chargers both run at 5V, so in order for them to work, the battery pack interfacing circuitry in the MB would have to activate starting at 5V, and there is nothing to suggest Apple would configure the circuitry to active at voltage lower than what the original charger is putting out.