It would be great if you could pick between an 'include' or 'exclude' mode., similar how Vray did (does?) this. Because if you only wanted to white out 5% of the model, it would be ridiculous to have to put 95% on the exclude list.
Also, I'm not sure if I would want it per-object or per-material. Each would offer challenges. It would also be worrisome on how this is saved in the model exactly.