> keeping a Windows, OSX, or Android device nearby while using my MorphOS system
> is not too high a price to pay as an alternative to wasting approximately half
> my MorphOS system's processing power only to render lower resolution HD videos.

As MorphOS doesn't support any power saving features on G5 systems (and most G4 systems), this only makes sense if you turn your MorphOS system off while switching to an alternative device for video playback. I for one find it too inconvenient to switch devices and turn my MorphOS system off just for watching one or more short videos in between.
Everyone who is concerned that much about energy consumption shouldn't even use MorphOS on a system with more than one CPU as each CPU, used or not, runs full speed with MorphOS. (That's the reason I desire an ASMP solution for my 2nd CPU, as discussed some months back.)