I was under the impression that -level 100x0% would produce the equivalent of -negate. This does not seem to be the case any longer (at least as of ImageMagick-6.7.4-10 and possibly earlier.
This is from Anthony's Users page at http://www.imagemagick.org/Usage/color_mods/#level where he uses -level 100x0% which shows what it should look like.

My tests with IM 6.7.6.8:
convert rose: -level 100x0% rose_level_100x0.gif
convert rose: -negate rose_negate.gif
