It depends. The majority of Cushing's horses can't be on a lot of pasture. That's because insulin resistance is often seen in the same horses that have Cushing's. However, some horses with Cushing's do not have insulin resistance, and those horses can be on pasture. Those horses are more rare, so if your horse has not been tested for insulin resistance I would be very cautious.
My mare with Cushing's does not have insulin resistance, so she is out on pasture during the day. I limit her hay and keep her body condition score at a 5 or less to help prevent her from developing insulin resistance. She also does not get any grain, and the hay is low NSC.
I'd think about what your goals are for this horse...with serious arthritis, DSLD and possibly Cushing's, perhaps your goal is not to keep her around for a long time but rather to let her be happy during the possibly shorter time she has left. If your goal is more about her short term happiness, then I would probably let her be out on pasture enjoying herself even knowing it might end up triggering laminitis and she might have to be put down. If she has three progressive, incurable, chronic conditions I'd probably lean that way myself.