Just so I understand - your PowerMac G5 will not turn on when you press the front panel power button. Is that correct?
But, it will turn on when you press the power button on your Apple display...
Is that also correct?
And, you also say that when the PowerMac does turn on, you still don't see any light near that front panel power button.
And, is that also correct?
Does that front panel light ever come on, or start flashing?
Did your PowerMac work to get OS X installed on your new hard drive?
If yes, does your PowerMac boot and run OK now, except for the power button?
The ? flashing on your screen means that there's no boot system, so I assume that you have not installed OS X yet. If you CAN boot to your installer after getting power through the display power, then you should be able to install OS X.
Last question - What did you mean by the 'board switch?"
Do you mean a power button on an old Apple USB keyboard?
That can't work - your G5 is too new for that.