I've used vinegar and baking soda successfully in similar situations with a pan. Dump a bunch of both - enough vinegar to totally cover all the burnt bits - and let it sit overnight or for a few hours, then more baking soda when you are ready to scrub it out. Good luck!

There might be a reason not to do this with a slow cooker, but my cleaning stubborn dishes trick is boiling water. It hasn't failed since the day I used it to remove burnt oatmeal in Boy Scouts.

I've used vinegar and baking soda successfully in similar simulations of a volcano.

In fact, you could combine that quintessential school science fair project with the way Richard Dreyfuss fucks up his living room in Close Encounters by tossing all the soil and shrubs from your yard onto your carpet and shaping it into a mountain. Then, you could hollow out the very peak of it and gently lower the slow cooker pot into the depression you've made (use a ladder.) Nice extra touches like tiny trees, cabins, boulders, 'horrified skier' and 'clueless USGS employee' figurines can be added to the slopes before you add baking soda to the lake of vinegar in your slow cooker the caldera. Be sure to film it and post it to YouTube for us!

Castille soap, like Dr. Bronner's? I've started using it for heavy kitchen clean-up, and I'm beginning to think there is not a single thing that Dr. Bronner's soap cannot take off. Just pour in a bit with some hot water and soak it more. (I know it's more soaking, which you've already done, but I really think the castille soap cuts through grease in a way that I've never seen anything else do.)

How tough is the interior? Rattling around several cubes of ice may help loosen up the gunk.

Time, heat, and slightly acidic fluids are your friends. 6 to 8 cups of water, 1/2 cup white vinegar (5% acetic acid), and 8 hours of slow cooking, and your "mess" should come out pretty easily, with a soft scrubber/sponge. Don't use metal utensils or anything that will scratch the glazed surface of the ceramic liner, like steel wool, the harder "heavy duty" scrubber pads (which often have pumice stone or other abrasives included that are harder than your pot's ceramic liner glaze), or Brillo pads.

In the future, consider slow cooker liners to avoid such issues entirely.

I'm trying taz's solution first (my, that was a nice fizz!!!), and if that doesn't work I will move on to paulsc's. As for the slow cooker liners, if I have to buy another product just to use the damn thing, then I'd rather get rid of it; it's cumbersome enough as it is.

Sometimes I have used heat, as well, with the vinegar and soda thing - not usually long, slow heat, just because I'm really impatient. But if it was an incredibly stubborn mass, and my first pass didn't work, I would definitely let it soak in vinegary heat for a long time.

Oven cleaner can be effective as well. Just be careful not to get any on the unglazed part of the pot.