Haha I stumbled upon this by mistake. When you have a full uber charge go into your spawn, pop it on yourself and quickly go to the character weapon configuration menu and switch any weapon that is not your needle gun. Once you do this you should go back in game with a permanent uber and a needle gun. You can switch to your bonesaw, but you cant change to your medigun or your uber will stop. Quite funny to run around in 2 fort bonesaw - spawn camping
Enjoy :)