Just a quick follow-up… I got this wired up and working, and I think it’s going to serve my purpose. Even with large uploads, the “input changed” event doesn’t fire until the upload is complete, which is a good thing. 
Being able to flag the file immediately on upload makes the difference. (I guess I overlooked the obvious initially.)
Also, I think I might create the “Orphaned Files” list on the user instead of as a global. That way, I could at least display a list of “abandoned files” somewhere in the user’s account settings - just in case the garbage collection misses a few. I can’t seen any downside to doing so right now, but lots of testing to do.
Thanks for the helpful input, @JohnMark and @help.
Now off to create “replace image” functionality! 