Finder generic icons for music/movies on second drive
Unfortunately the icons now appear to be hardcoded, it used to be the case that you could just name a folder Movies in your home folder and it would gain the icon but not anymore.

According to this: https://stackoverflow.com/questions/8380562/adding-buttons-to-finder-toolbar

DropBox are going to pretty extreme lengths to get their icon in the sidebar, using code injection. 

In other words, I don't think it is possible I'm afraid.