IO with port manipulation while not interfering with i2c
I don't think you need to worry. A simple piece of code:

void setup() 
{
 PORTA |= bit (0);
}

void loop() { }


Generates:

00000044 &lt;setup&gt;:
  44:   d8 9a           sbi 0x1b, 0 ; 27
  46:   08 95           ret


In other words, the compiler generates code to set that bit, ignoring all the other bits.