Here's my 2 cents.
Definition of "Weapon"
any instrument or device for use in attack or defense in combat, fighting, or war, as a sword, rifle, or cannon
I don't see, Hand, Foot, or Forehead listed.
Nor would I call a body part an "Instrument or Device", nor would I say that attack or defense is its primary use.
A chair is not a weapon, I can improvise it as a club, but it is not a weapon. Ditto a body part.
While it is true that some Foci can be done as Tattoos, Weapon Foci are not one of them.
Weapon Foci as stated on Pg.320 and quoted by others above ALWAYS has the form of a melee weapon.
CRB Pg.423 lists a whole page of "Melee Weapons" on the "Melee Weapons" chart. Hands, Feet, Foreheads are all not listed there.
While I can see someone tattooing their hands with magic, to me, that would be a Killing Hands or Penetrating Strike Qi Focus.
I don't see any way to make "your hands" into a "Weapon Focus" by the rules.