I want to do Philip Glass style poly-rythms with various instruments playing concurrently in different time signatures. Is there a way of doing this in cubase pro score editor?
Not in the score editor but try Indipendent track look aka local loop.
Set the loop to any value like say 18.104.22.168 to 22.214.171.124 and enable loop. Then you will hear the track loops the length while other tracks aren’t. Once you make a good loop, do “repeat”. Then the sequence is filled to the end of the part. Do this for other tracks and you can make such tracks very easily.
Maybe the function is also available in score editor? Please try.