I wonder if you could use some 10mm rope and a small prusic/carabiner as your bridge.
I do like the amsteel, but I don't believe it being ounces lighter is a deal breaker for me. It not being adjustable is a huge deal. Most people will get the new mantis with a set bridge and base their whole saddle experience off of the comfort from that particular bridge. I have found I like a shorter bridge. I would also like it to be adjustable. I just have to figure out exactly what I want.
I also thought about doubling my tree tether, but I don't like being unattached ever, so that's not an option for me.