After all the frustrations with the optoisolators last time I designed a board for the ACSL-6400, which is a quad optoisolator, and ordered some. Today they came so I assembled one and tested putting the CS, MOSI and SCLK signals through it. This one has 4 signals in the same direction, but there is also a 3 out 1 in version specifically for SPI. I didn’t use this because the ADS1299 has both DRDY and the MISO signals I want to connect.
Everything appears to work. I have a ton of extra boards, so if anyone is interested in buying assembled quad optoisolator breakout boards, please contact me.