Negative Artwork in Altium
You can use the 

File>Fabrication Output>Gerber

to generate the output of the required layers. Then on the generated Camcastic file you can go to

Tools>Film Wizard

There you can select the Negative for the layers and then you can also mirror the layers you want. Afterwards you just have to print the file. The printed file can be imported into Gimp and thus you can select the unused black parts and delete them to save the ink. I hope this makes sense.