What about a Druid who was using the Shaman subclass from the Advanced Players Guide. Shamans get an "Aspect of the ______" at 2nd level, which is considered a polymorph spell, but doesn't fully transform the Character.
So let's say you had a Bear Shaman 2/Monk 1 who used "Aspect of the Bear" to give themselves a 1d6 bite attack and 2 1d4 claw attacks. Could "Flurry of Blows" be applied to these attacks?
I'm afraid I don't have the APG sitting next to me to check and see if these attacks are considered natural weapons or not. I think it makes thematic sense, but I think it has the potential to be unbalancing. Add a spell like "Bull's Strength" or "Enlarge Person" and you're talking 25-30 minimum damage in a single attack.