Well...the matte glass might diffuse the laser beam or reflect it at weird angles, so it is best not to do it. Besides, the matte glass might crack from the temperature difference. However, I'm not entirely sure about this. I've never tried laser etching on glass before, but I think that the result isn't going to be ideal. If those panels aren't expensive, then you could buy two of them and experiment on one so even if the result is unsatisfactory you can still use the other one. Also, there might be some manufactures that are willing to customize the IMEI for you, so you can probably ask them whether it's an option or not.