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.
im now on my 4th or 5th slow cooker and i cook ALL my meat in it. If cooking a joint of meat i just put the meat in, nothing else(maybe a couple of onions cut in quartersbut literally nothing else). It makes its own juice, sometimes quite a lot. I have also tried jacket potatoes (just once) they were cooked fine but the skins were rather tough, im not sure if it was the slow cooker that made them like that or of it was the variety of potato, they didnt crack the pot and were totally dry. I wouldnt think they would be very succesful if put in with meat as the juices would make them very soggy.