You can do it by piercing the shell and inner membrane with a thick darning needle, then wiggling the needle around inside the shell. We used to do this when we blew eggs for decorating at easter. If you make a hole in both ends of the egg, you can blow the mixed contents out into a bowl, leaving the empty shell ready for washing and decorating. Don't know if that's any use to you. Why do you want to?
Thanks for the tips, was discussing this with a colleague who was planning on inventing a device if there wasn't one already; think he wants to save on washing up!