Slytherin and Polyjuice

I have very little interest in any of the HBP Polyjuice theories--Draco as Tonks, Narcissa as Tonks, Narcissa as Draco, Tonks as Narcissa. There is only one place in canon where I would be willing to consider it, and that's in the library when the hear someone nearby who's presumably Draco, but then Pince appears around the corner.

( Oh, those wacky gender-bending Slytherins! )