How can we pressure Apple into releasing their source code?

You vote with your dollars, and if you care what Apple is doing, you tell them.

Buy a non-Apple system, write to Apple, and let them know why you're not longer purchasing Apple equipment.

It's really simple, if you want companies to change, you stop giving them money (and you tell them why if you're no longer doing so). Giving them money tells them they're doing everything just right.

I don't think it would happen unless all the fans of the brand start understanding the importance of opensource. But knowing Apple, they would go bankrupt before they bend to the demands of what people actually want. As long as their customers applaud or defend every dumb decision Apple has made, they will never change.