There is nothing to gain for Ecuador by escalating the situation. Already they spent a lot of political capital and goodwill vs US/UK by hosting Assange in the face of overwhelming hostility, cracking practical jokes at the MET would only make things worse.
There is a lot of pen-testing fun to be had here. At the least you'll annoy them and amuse you, and at the best you cause them to expose the holes in their security capabilities.
I suppose there's the argument about how much the Ecuadorians are willing to risk annoying the UK — allowing him to stay is one thing, smuggling him out another.
Some of the shenanigans that went on with diplomatic bags shows that some countries are prepared to do this.
http://news.bbc.co.uk/1/hi/uk/1132544.stm
The Soviets using one to transport a tractor shows the bags can be big enough to get a person out. http://www.straightdope.com/columns/read/2234/is-there-such-...