It seems like this would be very programmatically difficult to do if everything is on the main workspace. We just need to make a couple routines for adding and deleting icons with their windowlets. You're absolutely right. Documents will not have a viewer in their windowlets. Documents are holding places for the data, which are transient and ephemeral.
Documents represent data that are being held in the filesystem somewhere.
So, the path is important. But we still maintain that documents can't change their association with a filesystem file. They should be treated as the Mac treats its icons. We should put the path in a label widget rather than an edit box.