I created a proseMirror-widget project to demonstrate how ProseMirror can be used to embed a number of content widgets (media, spreadsheets etc.) into HTML and how survey questions can be easily created. It also shows an alternate way to implement modal dialogs to prompt for attributes including an upload dialog and validation. Feel free to use or modify the source in your own project.
We are looking at the possibility of integrating ProseMirror into ThinkSpace, an instructional website devoted to helping students learn to solve real-world problems. In addition to the capabilities above, ThinkSpace also supports student teams, making the collaborative features of ProseMirror quite attractive.