Q: 2005 iMac G5 to Macbook Pro monitor setup: is it Possible?

No, that is not possible.

Actually it is but it is extremely complecated and involves an emulator called "Q"

I'm a very big fan of you Kappy!

I once used Q and QEMU quite some time back. Don't recall that as a feature then.

     As I said an extremely difficult process, you have to add a mod I'll link it to this forum when I find it.

I have a similar situation when a friend gave me his Mac Mini with a dead video output.

I used Screen Sharing to control it from my other Mac Mini that I am typing on now.  I think this is referred to as a "headless" Mac Mini and I connected an Apple dongle to the Monitor out jack that provided a composite jack, just so it would think it was connected to a monitor.

iChat supports screen sharing with Bonjour.    You could pull up iChat on both machines and use screen sharing that way.  Slow, but it would work.  Mind you aren't using the newer machine's processor to display the older machine's content.  That's all still controlled by the older machine.