This is a very interesting question. Unfortunately, I think the only one that would be able to answer it fully would be a lawyer with some familiarity with artlaw/commissions, copyright, and intellectual property. Also, remember this is the Internet. The copyright/IP law in the US is going to be different from the law in Japan, or Norway, or elsewhere...which makes the situation even stickier. I won't even get into the US-related copyright caveats when it comes to "parody" works and fair use.
I've seen plenty of folks on DA selling buttons, t-shirts, etc. at various conventions (and their own websites) obviously using someone else's original character (e.g., Inuyasha). A law is only worth anything if it's enforced.
Obviously, unlike a lot of other entertainment companies/people/organizations, Viz et. al. doesn't seem to be on the warpath to challenge people over their copyright. IMHO, I would ask one of the artists on DA who have been selling this type of work already and see if they are doing anything on the legal end of things with regard to this.