I have done something similar - I removed the stock webbing strap, trimmed a couple of lengths off this, and used it to make a webbing loop through the channel at each end (just gives me more versatility if I need to muck around with the suspension instead of removing it from the channel each time). The carabiner attached to my whoopies then clips straight onto this loop. I've just received 15 metres of Amsteel, so when I have time I will replace the whoopies with this.
I don't how Amsteel will go with the decender rings, as I've only ever used rings with a larger diameter rope. But as with everything hanging related, it is trial and error, and what works best for you. If the Amsteel was too narrow a diameter, it would be a simple fix to just use webbing tree huggers around the trunk, and then use that as a base to adjust and tie off at the right angle.
Could you post a few pics once it's done - I'd be interested to see how it looks.