That is indeed the file.
I misunderstood what you said about brightness before (I thought you meant brightness as controlled by OS X), and I have been able to easily reproduce this bug in both 10.6.8 and 10.7.4, including the part about all screenshots looking normal, as you mentioned. Since I'm not a graphics coder, I can't investigate it further, but hopefully someone who can will, and having your debug log should help.
Could you also make an "improved fs2_open.log" and attach that as well? You can follow the instructions for it that are in the same post I mentioned before, with one change: put the debug_filter.cfg file in the same folder where you found the debug log, rather than in Diaspora/data/.
Thanks for reporting this issue.
EDIT: I made an improved "fs2_open.log" myself using my iMac running 10.7.4 by starting Mission 1, flying through the launch tube, changing the brightness in-game to 1.10, going back to the mission, then exiting the mission and the game. The resulting log was so large (7.9 MB!) that I had to compress it before I could attach it. I tried turning off the more advanced graphics options (FXAA, post processing, soft particles), but it didn't make a difference.
EDIT 2: Also found that turning off all flags didn't change anything, but that if I turned to face the sun, the problem went away, although when I turned away from the sun, the problem came back.
EDIT 3: Confirmed that even with the default Diaspora flags, turning to face the sun temporarily fixed the problem.
[attachment removed and sold on the black market]