I saw this "tip" in Atmel's C programming app note (from a long time ago)...is this still viable, widely used or never used???...never heard or thought of using "spare" control registers until now. Seems a bit dangerous & sneaky (and a potential for mysterious brain busting bugs).
Unused locations in the I/O Memory can be utilized for storing global variables when certain peripherals are not used. As example, If the UART is not used the UART Baud Rate Register (UBRR) is available, and if the EEPROM is not used both the EEPROM Data Register (EEDR) and the EEPROM Address Register (EEAR) will be available to store global variables.