Got the opportunity to test a few other things.
You where right about ffdshow alone (using libavcodec of ffmpg-mt).
But, since I use coreAVC and ffdshow in the same time (coreAVC decode, send to ffdshow, and ffdshow perform upscale and rgb32 conversion).
http://img256.imageshack.us/img256/2421/tttwk.png (directvobsub has no effect)
What it mean is that it's no the link between bsplayer and coreAVC and therefore maybe coreAVC fault.
I also tried falling back in VGA, changing COLOR ICC profile, no change.
I also performed a massive modification of the video stream in ffdshow, removing any flag information (aspect ration/interlacing etc...) to be sure coreAVC don't say anything to bsplayer) but didn't worked
What’s left to test is changing software version (older CoreAVC, and bsplayer)
EDIT: Tested coreAVC 1.8.0, same problem (CoreAVC 1.2.0.0 failed to work under 7)