When I do my laundry at school, it's rare that I don't find a full machine of clothes (after the cycle has stopped). I just move it to a table and throw my clothes in. I've never had anyone complain about that, nor have I complained when someone did it to me. As long as you put the clothes on a clean surface, most people don't mind.
I like Cynthetiq's idea of throwing it in the dryer for the person, however, if someone did that to my clothes I would just assume they stole all my clothes. I wouldn't assume someone put it in the dryer for me. Now I know to look
I might add that to my list of nice things to do for people when i have extra change..