Nah, it must be someone else. Maybe another survivor of the academy bombardment.
Bastila is unlikely, she had her sith days after being molested by Mr. Iron jaw. Getting her to be sithed out again would not be as surprising. Well in fact there are not alot of "sithable" characters. HK is not "sithable" obviously, a Mission/Zalbar sith would look stupid. People like a Rakata would do wonders, though unlikely, and having the pazaak champion as a sith is... *cough*
Bao-Dur? Actually its a GREAT CHOICE. His fate is not mentioned by the old hag, and his death is uncertain. He would look gentle and unsuspecting. And his technological genius would make him the perfect suspect to cook up a droid planet acompanied by New HK series droid.