How to read SAA1064 7-segment values
SAA1064 provides commands for writing display values, but they are write-only and they can not be read back via I2C **.  Even if reading back through I2C were possible, you would have to implement a milti-master such that it doesn't interfere with the original I2C activity inside the heating system.  That may or may not be straightforward.

So, what can possibly done?

The O.P. mentions adding a microcontroller (μC), which would listen on the I2C bus.  There are existing products for doing this; they are already programmed to sniff and shouldn't require programming the μC.


Bus Pirate is a low cost Open Source tool
Total Phase Beagle is a commercial tool.  There are many more commercial tools like this.


However, I don't know how easy or challenging it would be to interface them to a Raspberry Pi.

Another approach would be to collect the number at the output of the SAA1064, which goes to the LED characters.  SAA1064 multiplexes between characters and that presents a certain challenge, because you need to know which character you are reading.  Ideally, you would use MX1 and MX2 as triggers and latch the values on the digital bus.  Then you could read them through an I/O expander.

** Details on pp. 5-6 in the datasheet.  In addition to the datasheet, I've got first had experience with SAA1064.  I've designed it into 2 commercial instruments.  To me, this lack of read back capability is a lack of feature.   But then, SAA1064 was an old (1991) part.
By the way, SAA1064 has been discontinued.