The choir teacher just revived new Chairs for her choir that are designed to promote proper breathing and posture for musicians. Having had a sore knee this week I tried siting zazen one yesterday. It was great, promoting good posture and breathing. They are Wenger Symphony Chairs http://www.wengercorp.com/Chairs/MusicPosture.html This may be a place to look if poor knees are preventing you from making it to the cushion.
Imsu