PLA Lost "Wax" Casting
TL;DR: It works, try it.
I finished off the furnace with another couple of pounds of refractory cement, and let it dry for a couple of days, then fired it slowly over a few hours to burn out the rest of the water. Things looked solid, so the next step was to try and cast something. Of course, my first thought was to “forge” some currency, and the results are below:

He’ll stand about 30" tall.
I’m making a Sasquatch next!
We’ve put a grainy video up of the project set up in our living room.
Maker Faire is next weekend so get your tickets soon!