Windywillow, I have never tried cooking JPs in a clow cooker, or cooking anything without liquid, but I would imagine if you want to be econimcal then it would be better to microwave the potaoes for a few mins first then put them in the oven to crisp them up a bit, or just do them completely in the microwave.
A slow cooker wouldn't give a very nice skin to a JP and would take absolutely ages to cook I would imagine, and could probably equal the same energy used as a microwave and an oven.