Why do you not want to put a PHP statement as a default? Long term, that's going to be a much better route and easier to manage.
Think about if you ever want to change that default thumbnail...you'd have to delete it, reupload it, and reset every post again.
Using a if ( !thumbnail ) { use this } is a LOT easier. It's really not much of a toll on your server either, if you're concerned about server resources. PHP is very fast when it comes to this.