Will bartenders hack your Point of Sale?
chompstack.com
chompstack.com
It doesn't matter if your POS is built on open source technology if someone has the ability to install new software onto it.
i.e. UltraSecureAwesomeClosedSourceOS is just as vulnerable to having new (rogue) software installed onto the device as any other OS. The hardware or firmware should have controls in place to prevent this or make it extremely difficult, regardless of what is running on top.