The recommendation to use HDMI with REW is pretty strong - if you want to be able to send a lossless test signal to any of the channels it is your only digital option. There is a great deal of experience in the various forums using ASIO4ALL to drive HDMI with REW. Using optical is not the same, especially for 5.1/7.1 channel setups as compression would be applied to the test signal.

You could use optical for stereo left and right channel measurements and calibration (+sub after crossover) as stereo is still lossless over SPDIF. You could manually switch the left or right pre-out to your amplifier's other channel inputs to drive individual surround channels, but that sounds exhausting.