Preliminary tests are done, and I'd have to say that the answer for a doll with a lot of sentimental value is probably "no, dremel on porcelain doesn't work". The long answer turned out to be more complex than I thought it would.
I'm still prepping the pictures (for some reason, my camera insists on always taking HUGE images that have to be post-processed down to a sane size), because the results are interesting.
The dremel DID actually remove the paint from the test head, and did so even more easily than on any Barbie to date. But it did not do so without affecting the porcelain. I say 'affect' here and not damage because that's where it gets odd. It doesn't feel scratched in any way, it didn't chip or crack (even when I worked dry). But the area I removed the paint from now has a slightly softer/more matte finish that looks just a hair lighter than the surrounding area. I think the color difference is an optical illusion caused by the surface sanding as the color of the porcelain in the head I'm using goes right through the piece. It kind of reminds me of sanding the shiny/hard plastic parts of vinyl dolls to either get them to take paint or blushing or just to remove the 'unnatural' shininess. The area doesn't feel any different to touch than the rest of the doll.
It also means that if a dremel IS used to remove paint from a porcelain doll, the dremel would have to be used over the entire surface, not just the painted part, and then the whole thing followed up with a very fine grit sandpaper (240) to soften the effects. I'm going to continue the experiment and try that on the test head to see if it results in the skin tone looking even again. If so, it'd be a method that /could/ be used on a doll /if/ one wanted that effect on the doll's 'skin'. (I actually kind of like it, it looks more natural somehow.) And I would recommend getting a muggable test doll from a thrift store and trying it at least once, because it really is a kind of cool effect - just not one you might want for your gran's doll, as it's a pretty permanent all-over change...
I'll describe exactly what I did step by step when I get the pics up.