What kind of closed source / commercial software can be fuzzed on your platform? E.g. how would you fuzz something like Adobe Lightroom or Autodesk AutoCAD there? What kind of reports would you provide?
You won't get the same kind of results that you could by writing your own harness, but it would still be possible to find crashes, extreme memory usage or timeout bugs. Using something like libdislocator [1] would allow you to expose certain memory bugs as well.
[1] https://github.com/mirrorer/afl/tree/master/libdislocator