Histogram measurement in single spot
The first thing I'd be looking at is Processing. There's a library that lets you read video, there's an example here that shows how to to sequentially grab frames from a video.

Then (instead of displaying the frame like it does in that example) you need to find the channel values for the spot which you could do by copying the frame to a PImage. Then the value of the pixel (x,y) is found in the PImage property pixels[x,y]. Finally use createWriter() to output the data to a file. You could even get fancy and use the mouse to control your sample point, and so on.