Generating video thumbnails preview / contact sheet in batch, color issue
So here is what I found out, if anyone's interested (however unlikely this might be, considering the poor popularity of that question ! :)) :
– Scorp Video Thumbnails Maker gets the colors right for Bt.709 footage with its “Engine 2” rendering module (not the other three with the current 11.0.0.3 version) and the CCCP codec pack. I created a thread about that issue on the support forum for that tool.
– I managed to tweak VCSI by adding "-vf", "zscale=matrixin=709:matrix=709,format=rgb24", to each ffmpeg call, so that it preserves Bt.709 colors; it's still slower and less convenient than Scorp VTM (and that modified script won't get accurate colors for Bt.601 footage so both versions would have to be installed and used accordingly), but it's free.
– Since I created this thread I found a few other tools which can perform this task : StaxRip (which has an internal thumbnails preview generation module, in addition to the older MTN), PotPlayer, BSPlayer, Auto Movie Thumbnailer. The latter is based on MPlayer, and is supposed to give control over the rendering engine, but I couldn't get it to use Bt.709. Otherwise it's a good free dedicated GUI alternative to Scorp Movie Thumbnails Maker, with many customization options.

I also created a thread on VideoHelp regarding that specific issue, and another about the color accuracy of screenshots in general.