Just register oUF's AltPowerBar element for the pet frame too. You could test this with
http://www.wowhead.com/quest=28733/basic-botany too, as it is more accessible.
When the player is in a vehicle, the pet frame holds the player unit:
For the pet frame then: self.unit = "player", self.realUnit = "pet"
For the player frame: self.unit = "vehicle", self.realUnit = "player"