Yes, I’ve been using a 29" tube for almost 2 years. It makes a big difference in weight and cost. You need to be very careful not to pinch the tube between the tire and rim when mounting the tire.
Using a 29" tube in a 32" should be very simple. BTW, in case anyone missed it Ben has been riding his 32" for about a month: The Missing Link: A 32" unicycle, à la Walmart and UDC