YES, please add it! It would be extremely useful! I've been searching for this option but the fact that I have to add every channel I don't want changed manually is very annoying.
This has now been added. Click the DeArrow logo in the top right of your browser and click "Channel specific settings" when you are on a video of someone's channel
If you set your profile to replace thumbnails, but set the default settings to replace nothing, it will only replace thumbnails for the channels in your list