I am not a lawyer...there's your disclaimer.

WotC being what it is (a subsidiary of a major publicly-listed corporation) I find it hard to believe that it has any artists doing work that is not classified as "work for hire". If that is the case then it, not the original artist, holds the copyright to the art and can do with it (or have someone do something with it) whatever they bloody well please. That includes creating derivative works. In fact, they do this all of the time...every time they re-use an illustration and crop or collage or sepia-tone it or whatever.

It may be that M.S. is big enough to command a different sort of deal...or i may simply be wrong about how Wizards rolls. If I'm right about the WFH thing, however, then your project may be okay. If you're doing this with their permission I'd suggest simply contacting them and getting their legal department (or Hasbro's) to rule on the matter. That's the simplest, safest way. My suspicion is that, unless you yourself are under contract to produce these things, that they will tell you that you're on your own and that all liability rests on your shoulders.
M