Midway:
it is only this past part with the PCF what makes it hard to find out.
No it is not.
It is exactly the same technique that is used to make individual bits change in shift registers. It is exactly the same technique that is used to make a digitalWrite() function work on individual pins of a port on the arduino.
It is a very simple concept which you seem to be making heavy weather out of.
The concept is you have a variable, you change only one bit in that variable and then you write the whole thing out to your peripheral / port / shift register. The bits you have not changed in the variable do not change in the output. The bits that you have changed in the variable change in the output.