Just to check: Did it turn out to be Sirius/Peter, as referenced in chapter 42? I notice that the phrase "artistically complete" appeared in the notes, so I'm guessing yes.

Edit: I was initially going to suggest something involving either bestiality or Animagus-form sex; possibly Ron/Scabbers, some combination of Marauders, or Luna/Thestral. The part about only finding one prior example made me think of Hagrid/Dobby.