Actually, the Tumble skill says that "obstructed or otherwise treacherous surfaces... are tough to tumble through." Not impossible. And if you can tumble through a square occupied by an enemy, why couldn't you do the same through an ally's square?
You must have a clear path toward the opponent, and nothing can hinder your movement (such as difficult terrain or obstacles). Here’s what it means to have a clear path. First, you must move to the closest space from which you can attack the opponent. (If this space is occupied or otherwise blocked, you can’t charge.) Second, if any line from your starting space to the ending space passes through a square that blocks movement, slows movement, or contains a creature (even an ally), you can’t charge. (Helpless creatures don’t stop a charge.)
That's why you couldn't do the same through an ally's square. Specific trumps general. If you come up with an explicit exception to the no charging through allies rule, then you can do it.
That's why I said "The criteria is either A) you cannot charge over difficult terrain or obstacles as stated in the PHB, so no Jump or Tumble checks (unless there is no difficult terrain or obstacles, but then you wouldn't need to make the checks), unless B) you have an ability, such as Acrobatic Charge, that allows you to do so."
