Hmm, but I am spawning a petframe already.
Disabling the PetCastBar does the trick though.
Code:
--disable the vehicle castbar
PetCastingBarFrame:UnregisterAllEvents()
PetCastingBarFrame.Show = function() end
PetCastingBarFrame:Hide()
Maybe you can test it yourself at this daily quest (Wyrmm temple defense). The player is switched to the pet correctly but the petcastbar becomes visible upon that point.