I don't believe there is any rule in the book about when a player can touch a live ball that is not a forward pass. The forward pass rule is in place to prohibit a player from gaining an advantage by leaving the field of play and then re-entering to catch a pass. For example, a player isn't allowed to use a person on the sideline as a "blocker" to screen himself from a defender. To put it another way, this makes it so the defender can rely on the sideline as the absolute farthest a receiver can travel when trying to get open.
In the case of a loose ball (fumble), there's no realistic way that a player could expect to gain an advantage by leaving the field of play, so I believe it is highly unlikely that any rule would exist that would prohibit someone (offense or defense) from taking possession immediately after re-entering the field. Of course, the player would have to be completely inbounds, or else his touching of the ball would end the play and possession would revert to the last team who had the ball.
That is my understanding, but if someone wants to actually read the published rules and correct me, or add to my comments, please do so.
