OK, now that it's probably too late, there is a tool buried in the Effects tab that will work.
Effects, Image Control, Levels.
In and Out are 50 and both tolerances are 100. Change the Gamma number to 1.2.
The images should gracefully get darker and do it exactly opposite the way that FCP made them brighter. I could find no errors.
Do Not Use:
Effects, Image Control, Gamma.
It's very simple, but there is a messy assumption made with that tool and it doesn't work right--or it does something I didn't like.
Sorry I was so slow on this. Gamma controls are a total mystery to most Mac people so I had to design test patterns and tools to figure out what I was doing--or what it was doing.
What *I'm* doing is still something of a mystery.
Koz