Wouldn't get too hung up about hum cancelling until you try it out with the Tonerider J Bridge PUP. Tend to think it will be all good so long as you follow the YB4 wiring diagram on the main PBG site.

Was jamming with a bloke before xmas who was playing a 'Reverend' guitar that was fitted with 3 x P90's and when he was touching the strings no buzz or hum but the moment he took his hands away it was hissing like a snake. Doubt the setup you have planned will be as bad as that and he was quite happy with the axe saying it was all part of the character of the thing.