So this is one of those crazy ideas you get while laying in bed late at night, and as it turns out I'm not the only one!
Instead of making black carbon "glop" and printing that under a negative on white paper, you make white goo and print that under a positive on black paper. Voila!
Here are some other discussions on this topic, and I hope some of the carboneers here will chime in.
(you know who you are! )
If you can find any more resources, or examples, please share them. I think this could be a very cool way to make carbon prints from any positives you might have lying around.