I'm involved in a game where the following scenario came up. The group needs to get someone inside an NPC's home to do some magical snooping. They wanted to somehow enter without encountering the "crossing a threshold uninvited" problem. While discussing ideas, the question arose about whether the following would work: Let's say that somehow one of the characters could safely transform himself into an animal small enough to hide upon the NPC (let's say a flea on a jacket for the sake of the scenario). If the NPC walked across the threshold of his home while unknowingly carrying this character, is the character considered invited in? The rationale was that the home owner (the NPC) did the carrying, so the character should be considered invited.

Please assume that the transformation is safely possible by the character. Whether the transformation is possible or how much refresh the character mush have to be able to do it isn't the point of my question. Also please assume being uninvited would either disrupt things enough to either cause the transformation to end, or limit the character's abilities enough that the magical snooping would not be able to happen.