Acrylic is a plastic and as such is not conductive. The capacitance touch screen works by your body’s natural conductivity. Your finger touch disrupts an electrical field and that disruption is measured to determine where you touched it. The acrylic mirror is an effective insulator and will prevent your touch from registering.
If this blog/advertisement on photo booths is accurate, they use a Microsoft Kinect to perform motion tracking on the subject. Now adapting that would be an interesting RPi project!
For more information: How Do Touch Screens Actually Work?