Welcome to WetCanvas.
I think you are right about putting in a bit at a time and layering it.
It would easily support itself as it dries into a solid, dense piece of Styrofoam.
One problem with the product that I used is that if you don't use the whole can in two hours it binds itself up and can't be used anymore.
The directions say to lightly spray on water to help it dry faster
The next one is a sketch of how I attached the body to the legs.
Drill a hole in the top of the body as a starter and stick a dowel rod all the way through.
Drill a hole in the leg frame.
Attach the two together with the dowel rod.
This would give enough stability to hold the two together until I could add more layers of papier mache'.
Well things often don't work out as planned
I am banned from using power tools, so Brian (my husband) volunteered to drill the holes.
I held onto the poor Firdemu's body who.... still headless.... hadn't a clue as to what was going on.
As soon as he began to drill all the undried 'Great Stuff' that was inside and under pressure spewed out of the top.
The spinning of the drill distributed this gooey sticky foam stuff all over my studio before we had a chance to stop.
Here are two pictures of 'The Ooze'