From what i've always understood, if he is transported in a car together with a diplomat, the UK police are still not allowed to stop them. So he himself doesn't have to be a diplomat.
The Ecuadorian embassy isn't a huge building with it's own garage, it's a flat on the first floor. If you can get a car in to a first floor flat you may have a plan.
Even if that weren't the case he's got to get out of the car to get into the plane at which point he's once again on British soil and can be arrested.
Plus really, how much effort at the Ecuadorian government going to go to to piss off the British, American and Swedish governments? What they're doing now is following procedure, what you're proposing would be far more active.
So he jumps out of the window of the embassy into an open car beside an Ecuadorian diplomat, which is then driven into the back of an Ecuadorian cargo plane which then takes off for Ecuador. Realistic? No, but I'm sure the idea fits in just fine with Julian Assange's inflated self-image.
