Fly me away ...
It would be interesting if Ecuador were to engage a helicopter and a ship (do they have a navy?) - fly Assange off the roof of the building to a waiting ship offshore. Then the Brits would be faced with the prospect of a military confrontation to prevent the ship from leaving territorial waters (actually if they're 12 miles out, even that prospect goes away once he lands on the ship).
This could certainly be done. Even if the building is not set up for helo landings, they could use the rescue basket method. Of course there are certain issues with the UK air force as well. But would UK actually shoot down an Ecuadorian helo, just to catch this meatball?
As a bonus, the movie rights to the rescue would probably fund Wikileaks (or Assange's new Ecuador hideaway mansion) for decades.