Thank you for the feedback - this is exactly the kind of thing I'm looking for. You have to drag the icons from finder, not from the dock. Does that work?
Lots of people are also suggesting hiding the other apps. I might try to implement this as an option, but what I'm going for is de-cluttering the command-tab app switcher, and I don't think hiding the apps will achieve that. However, this feedback is inspiring me to see if it's possible to hack the task list, or whatever it's called.