Materials Sculpting a whole bjd with Cosclay?

I haven't done it, so I can only speculate. Feel free to ignore this post if you are not interested in that.

Even regular polymer clay doesn't hold up well under high pressure, so it's not an ideal material for a OOAK doll that is strung like a BJD. Typically polymer clay is used for originals to be cast later in a sturdier matreial, or for detailed parts such as hands and heads for less playable decorative art dolls, using other materials for the main body.

For casting:
Look into what it is that keeps the cosclay flexible. Is it some kind on plasticiser that doesn't fully envaporate during baking? If so, I'd worry that it would interfere with the primers and later the silicone during mold making. To make thing worse, this is the kind of thing that may not be noticable until after some time, such as silicone molds breaking down and wearing out a lot faster than usual, or casts made in those molds showing more microbubbles or other defects than usual.
It would take extensive testing and a lot of time to figure out for certain.

For a OOAK ball jointed doll:
I'd worry that the parts may warp and eventually crack with time. You need a lot of tension on the elastic to get a doll that behaves as well as a regular BJD. Lighter tension, even if the egineering is good, won't allow the doll to hold it's arm up in the air or it's leg out when sitting on something. You'd essentially get a loosly strung doll than can be elegantly "draped" on a doll sofa, but not one that can stand on it's own or be posed for everyday actions.
Also, how does it age? I know cosclay is a quite new material, so it may be hard to know for sure, but look into it all the same. If there are still plasticiers in the material, these will eventually escape. Best case scenario, the pieces will simply become stiffer, but they may also crack as the material shrinks or even start to break down completely. Basicly, all the same things that can happen to porly baked regular polymer clay.

If you try it out, let us know how it goes! It's always interesting to learn about new materials.

 

That sounds like a good plan.
The longer I've done any craft, the bigger appreciation I get for material tests and small test projects.

Best of luck and let us know how it works out!