I would assume labor, not simply the cost of returns, is a huge factor for them. Considering that, a packaged a solution that works out-of-the-box with minimal setup might be a prerequisite for adopting your suggestion. For example, you could build your suggested solution in a manner that works with the Pinebook Pro's built-in camera and create a Debian package that can be installed alongside and easily invoked from whatever burn-in pipeline they use. Then they can just point the laptop at a mirror during burn-in.