back in the day when i had both FCP5 and FCP6 on my system,
i got acces to the ProRes codecs in FCP5.
yes, i could create ProRes timelines!
i did quite a bit of Compositing in a FCP5 project using ProRes timelines
there was a gamma shift in the image between playing and not playing:
from memory, when not playing the image got a bit darker.
i wasn't doing any Colour Correction, so it wasn't a problem
i have no idea if the same thing will work with FCP5 an FCPX
nick