AVRDUDE: Write EEPROM bytes without file
The avrdude write command (-U) knows an 'immediate' mode (see the manual), which allows you to specify the bytes to be written directly.

The syntax is like

avrdude -U eeprom:w:0xff:m


But this mode doesn't allow you to specify the address of the byte to be written, so I presume it will start writing at address zero. This works great for fuses (where instead of 'eeprom' you use 'hfuse', 'lfuse' or 'efuse'), but not so much for real memory.

But you can always create a hex file (e.g. INTEL Hex records containing just the single byte with the address you need to change.