hi,
I do want to use the I2C hardware - not for saving CPU ressources, but for
saving code size.

may I say that usage of the I²C is not a good idea when you have lack of code space. This interface has much more complex protocol than, for example, SPI or UART.
For example, even Master transmit mode has about 10 possible states which must be processed correctly. So think twice before come into troubles with "not enough memory" etc.

Regards,
Oleg