I've seen people discuss using Mac ADB keyboards with ADB NeXT hardware here but there were questions about the power button. Mac ADB keyboards DO have a soft power button on them. The button is labeled with a triangle pointing to the left. On most Apple keyboards, this button is at the top, above all of the other keys and is not shaped like the regular keys.
Here are some examples:
Likewise, the Power Button on the NeXT ADB keyboard will also work to power on an ADB Mac.
Another interesting fact, next equipment uses Apple branded ADB transiever chips. They are 100% compatible at the logic level.