Is it possible to use GPIO pins as chip select for SPI and leave the chip select pins PCS[3:0] available for other functions? From what I have read in the documentation it seems like the programmer is required to use the specified chip select pins.
yes, it is possible to use GPIO as a chip select and toggle it manually when needed. But if you use SDK, be aware that the SPI drivers do not support this customization. If don't use the drivers, there should be no problem.
You can use normal GPIO pin as chip select. I tested like that it is working fine. You select any PCS[0:3] pin and configured that pin to GPIO. Which PCS pin you are selecting, write that value in PCS field in TCR register in LPSPI.