You can send the following commands to out endpoint control the LED.
1 byte: 0x80, to toggle LED
2 bytes: 'F','T', to toggle LED
2 bytes: 'F','O', to turn LED on
2 bytes: 'F','F', to turn LED off
You can use the following schematic and layout to add your own feature.
Either Low-Voltage Programming via USB pins or High-Voltage Programming on RC0 and RC1 can be used to update firmware.