I think that was my post about the Powermac G5 sound issue - if so, glad it helped.
Have you seen this post about trouble with nvidia cards and 15.10 ? I recently rescued my Powermac G5 from someone. I discovered it was having a hardware problem that was preventing any of the bootup key commands from being accepted (startup disk menu, force CD boot, target disk mode, open firmware access, etc). Turns out it was the video card, which otherwise had no symptoms once OSX booted. It was an ATI Radeon 9600 which I replaced it with a Radeon 9650 I found on ebay for $10 delivered, knowing I wanted to avoid the nvidia cards for the sake of installing ubuntu-mate. If you can’t find a workaround, it may be worth swapping out the video card to an ATI to get away from the problem. I can send you a message with a link to the listing if you are interested - let me know.