Is it feasible to try and classify an enclosed space as being a. Very crowded b. Less crowded & c. Sparsely crowded using image processing?
This sounds feasible (and fun!), but it will be hard to get very accurate.  My guess is that, like your tags suggest, this would be a combination of edge detection / space detection to get a measure of density and face/hand detection to get a measure of human content.  You could then train up an SVM or RandomForest model on the data, tweak, and repeat.