No sorry not heard of wheelchair swings but I know in our local park in Cardiff run by the council they have an accessable roundabout. My son Ross never liked swings when he was younger but did go on the roundabout a few times. He used it in his manual and electric wheelchair and he would drive onto the roundabout. In the middle was an enclosed area and he remained stationary whilst the other children operated the roundabout.
Sorry cannot be more help