Trying to get the natural lighting to illuminate only the mug's rim, and the steam, but also with a long exposure.

I'd say you need to get rid of that area of blown-out light at the bottom right and that will allow you to increase the exposure everywhere else without making that area bigger and more obvious.

Thanks for the suggestion, I might try it out in post processing, or block and shape the light source next time I compose a similar image.

"Natural" light photography many times needs some assistance. Not from artificial light (side Q, how can "light" be artificial???), but by using reflectors. Try a reflector card on the camera side of the subject for the steam, another one for the rim, out in front of the mug, left and out of the camera view. You also need to separate the dark mug from the dark BG. You may need a third reflector to the right of the mug to put a rim of light on that side.

Thanks for the ideas, I have one 36 inch reflector and a light stand, and a white folding display boar that would probably serve as a really big bounce card as well.

K-5 with Kit Lens

Lightpainting requires manual settings as well