Yeah, I'm working on (just barely, over the past year) a way to configure the behavior using control requests to the HID and saving the configuration in EEPROM.

Actually writing new code to the device would be a bit harder...not impossible, mind you. It would be a variation on the USBTinyISP code, and you'd need about 2300 bytes for the bootloader so it would be a good idea to move up to the attiny85.