That is actually quite smart. You are correct there would not be any side car files for those you did not edit. However if you import files and have it automatically include a copyright for example and the auto save feature was turned on, then it would not work.
I am adding that for anyone else reading this thread.
Problem with having an automatic solution like you mentioned is Lightroom would not be able to tell the difference between unwanted files and those that were just put on your drive that have not been imported yet.
However one solution would be to add the rejected tag to the metadata, the lightroom could search for that tag.
Just so you are aware, this is a user to user forum with very little interaction from Adobe. But you could goto the photoshop.com site and use its request forum to request this option. I think it is meant mainly for photoshop, but lightroom is suppose to be a photoshop product, so...
Here I found the link for lightroom in that feedback forum. Recently active topics in Photoshop Family about Photoshop Lightroom