This seems like an obvious idea so I bet someone has tried it before:
For some mortise pin tumbler locks which operate a cam, the keyway opens up at the back of the cylinder and would seem to allow threading a tool to the inside of the mortise and possibly pushing what the cam is meant to push. Couldn't this be used to manipulate a lever which would unlock the door?
I'm thinking some springy flat metal which would want to bend towards the lever might work.
Although this image is small, you can see what I'm talking about:
http://comebackwithawarrant.com/Mortise.jpg
Here's another mortise with the cylinder removed:
http://comebackwithawarrant.com/Mortise1.jpg
What's the problem with this idea?