Of course, you can keep the trombone and the left arm up in a playing position, with continually tensed muscles. However, if you can avoid this, the important question is, is there really any sense in keep on doing it? Would the playing be better in the long run if your upper body could stay more relaxed?
The ERGObone helps you to keep a good posture and breathe more freely and efficiently. Would it not be better to lean, or even hang, your left arm on the trombone rather than keep it continually tensed up?