I can't find in now, but Martin Evening had a post on a preset for a negative. I can't find it, maybe some else has it. The approach was to create a preset in light by importing the negative into photoshop, flipping the curve so that it inverts the image, so the line goes from upper Left to lower right.
And then, you, uh, er... geez this is killing me. Help Help! What's the next step?
Sorry that's as far as my brain can go.
UPDATE: Found it!
http://lightroom-news.com/2''9/'4/13/creating-a-lightroom-invert-tone-curve/
The default Lightroom Develop panel settings don’t include an invert tone curve for converting a negative image into a positive, but it is possible to create one of your own. There are several ways you can do this and the method illustrated here in this movie is the simplest you can use to create a custom invert setting.
This tutorial came about in response to a photographer who wanted to photograph negatives with his digital SLR and process them directly in Lightroom. This is the solution I suggested, but as you will see, you do end up in a situation where all your major tone edit controls work in reverse, so you have to use the Blacks slider to control the highlight clipping point and the Exposure slider to set the black clipping point. However, the ability to create this type of setting for use in Lightroom does extend the range of Develop settings and what you can do creatively in Lightroom by applying invert tone curves to produce creative effects such as the
Smoke presets creates by Sean McCormack.
Reid