I know Genji's deflect can bounce off basic attacks, but can Genji deflect Brightwing's polymorph ability?
No he can not. Simply put: Unless the ability in question is a projectile Genji should not be able to deflect it.
And to clearify I want to mention, that "deflect" for the abilities that can be deflected simply means, that genji is unaffected by them, and not like many people want to believe, that you return the ability to the sender. (Information might be partial wrong, since I haven't seen Genji in any match for a few months)
According to official's Deflect ability description:
Channel for 1.25 seconds, becoming Protected and blocking damage. Any damage blocked while channeling causes Genji to throw a Kunai toward the nearest enemy, prioritizing Heroes and dealing 55 damage. Total Damage Deflected: N
Means that the only thing that Genji can Deflect is damage. He can't Deflect stuns, hooks, slowing effects, silence, or any other crowd control effect like sleep or dance, or polymorph, etc...
If the effect is Stun+damage like Uther's Hammer of Justice:
Deal 109 damage and Stun the target for 1 second.
Then Genji will deflect the damage but will be stunned anyways.