Let me get this straight. You want to take Weapon Focus (<one particular form of natural attack>) when you don't have a natural attack ? I don't think that works, unless you have a reliable way of getting a natural attack (an item or an at least once a day spell/ability).
That is, you're proficient with any natural attack (forms) you have unless the method you used to gain the natural attack says otherwise, but you're NOT proficient with any form of natural attack that you DON'T have, unless you have a reliable way of getting that natural attack.
Example: A kitsune can gain a bite attack by assuming kitsune form. She'll be considered proficient with that bite attack even if she's in human form (where she can't use it), but will not be proficient with a tail-slap natural attack unless she has some item or spell that reliably gives her one.