Wow, that’s a use I hadn’t considered!
I think you can do what you want by:
- Window > Themes > Edit Current Theme (that will show theme editing sheet)
- Change both the “Page Background Color” and the “Window Background Color” to be transparent.
- To do that click on the color well next to the color label, and then slide the opacity slider in the color picker to a transparency level that you want.
Then play the video in a different app in the background, and it will show through your transparent WriteRoom window.