Sorry for the lack of updates, aside from work (which limits the amount of time I have to devote to this guy)... a lot of the process of cleaning the seams after removing the torso from the cast is slow; a little bit of work on the seams... fill some gaps with latex... wait for a day or two for latex to cure... do a little bit more work.
So! I now have a torso. The inside is filled with expanding foam (there are sections I hollowed out, as I noticed even with light foam, he is still pretty heavy). PVC tubing was inserted in key points in his back, his which were then attached to a harness.
The harness itself has an acrylic sheet for a base, then wrapped in the backing of a backpack I dismantled. I chose the backpack material since it has nice airflow and a little bit of padding... it should keep me from getting too sweaty, but stay comfortable. Right now the shoulder part of the harness is a little high, so I have a lot of weight pressing on my upper shoulders/neck area. I may move these shoulder straps lower so they rest on my shoulderblades. I'll give him a few more test runs to find the best location to distribute his weight.