For something like this Tomato Jammy Sauce could you adapt it to a slow cooker?

You can't operate a slow cooker without the lid on. It really does require the closed environment inside to prevent the poor element from overheating to compensate for the heat lost to an open top. And using it correctly with the lid in place will not work because there isn't much evaporation, which is needed for this dish. However, you can make this recipe in the oven in a heavy dutch oven or other oven safe casserole dish. You won't need a cover, and the excess liquid will be able to evaporate. I would suggest setting the oven to about 275, which should get you a slow simmer. You can always adjust the temperature to suit your oven, so you can maintain that slow simmer.