Could I use Stoneware (from a slow cooker) in the oven?
Well after doing some searching, I found that the HB model I have does state the stoneware is microwave and oven-safe (the lid is not). Most of the other brands appear to be oven-safe as well. For what I am doing, I do not believe this will be a problem as I'll be moving it from the slow cooker into the oven for browning purposes. I will definitely use the recommendation of having a pan on the bottom of the oven, "just in case".
For anyone else that may be curious, check your model's manual or view the manual on the company's website to confirm if the stoneware is oven-proof/safe.
I'm not so sure that they are all safe. A slow cooker barely gets food to a simmer, which would be in the neighborhood of 200-250 degrees.
If you are going to try it, I would recommend putting a nice, deep roasting pan on the lowest level of your oven to catch the disaster that might happen if it should crack. I would also put a cold stone ware item into a cold oven, not a pre-heated one. Pizza stones will crack if you put a cold stone into a hot oven, so it is likely that this could affect your slow cooker liner the same way.
Good luck and let us know how it turns out.