How to read 10-bit RGB values from HDR video?
The ProRes input will have YUV encoding, not RGB. If you need a raw dump, save as rawvideo.

ffmpeg -i in.mov -f rawvideo raw.yuv


Note that bit depths which aren't multiples of 8 are still stored in data layouts which are, with padding. So, if ProRes is yuv422p10le in limited / studio range, the 10-bit luma value for a white pixel will be 940 or 1110101100 but will be stored as 16 bits in the file: 1110110000000011 where the order is little-endian.

You can however save as RGB 10-bit (with padding),

ffmpeg -i in.mov -pix_fmt gbrp10be -f rawvideo raw.rgb


where the green plane is stored first, then blue, then red. The above is for big-endian so the byte order isn't reversed.