Looks like that could be an option, and Table 9 reset defaults show it is indeed a good candidate and in default setting can accept input from the SIF and output it on SCART1. From there you feed the audio into the Micronas as the Mark 1 stereo decoder or mono hookup does via pins 56 and 57 of the Micronas.

You know, it's a shame the STV8216 outputs it's I2S sound data in Master mode only (refer part 6 of the document) otherwise it would have been a good replacement for the Micronas and solve everyone's sound issues... If it could have been configured for Slave I2S and someone was willing to write a version of palmod for the different I2C control commands and the necessary adaptor board (it isn't a simple "drop in" pin-for pin Micronas compatible replacement) then it would be an excellent chip to use in place of the Micronas altogether as it has both SIF sound input for use from the tuner and analogue sound inputs for use from the rear A/V input(s). Close... but no cigar