It would be extremely useful to spray objects like with the spray tool in Inkscape. Is anything like it in Blender?
Using a weight group, you can determine the arrangement of particles, and you can use a group of objects to be the particles. You are basically painting them in with the weight paint, and you can control how many of each of the objects that are used in the particle tab.
You can use the grease pencil to sketch where you want scattered objects and weight painting is almost exactly what you are talking about I think.