I intend to use the SC18IS602B to control a multiplexed line of LEDs using i2c. For this, I would need to use three GPIO pins to control a 3:8 demux chip and one GPIO pin to toggle the latch enable signal of a shift register. Then I can use the MOSI and SPICLK pins to sink data into the shift register. According to the SC18IS602B datasheet, as far as I can tell, this should be fine.
However, in practice, it appears that attempting to write SPI data to a slave will not work if that slave's slave select pin has been set for use as GPIO. That means that enabling all four GPIO disables the SPI output. SPI data can be sent to any combination of slaves, but as far as I can tell by the datasheet it can't be sent to zero slaves.
The datasheet claims that there is up to four GPIO and makes no mention (that I can see) that using all four disables SPI output. Is there something I'm missing here?