You will have to do more research but I thought there was a company that was redoing all the gears in the r154 an making them straight cut gears on the other forums. Maybe you may be able to find a reverse gear from them.
Here are a couple contacts that were helpful when I was looking for gears; both are on Supra Forums though.
SnkPoiSon - He works in the parts department at a Toyota dealership in Chicago; he was able to find the gears I needed that no one else had. Great customer service and went above and beyond to help me out.
supra_kid1 - He doesn't work for a dealership, but he has a used R154 that he is parting out and might have the reverse gear. He is a nice guy and was responsive