make a device for USB ejection
A MCU with USB interface can be chosen. The button will be a interrupt to the MCU. The MCU will be connected to the PC as a slave. Whenever the user presses the button (which is interfaced to the MCU) the associated firmware can raise a request to the application running on PC. The application later can handle the USB safe removal process. 

This involves both hardware and software design and hence might not be efficient for quick prototype.