At the risk of having a corner torn off my Man Card......
My torque wrench came with a little instruction book, and I actually read the directions. OK, let the jokes begin now....
The instructions say that an extension will NOT throw off the torque measurements as long as
it's in line with the socket and the connection on the wrench. So as long as you have a 90 degree angle between the handle and the end of the socket, you're golden. Where you get into trouble is if you use one of those "knuckle" type connectors between the wrench and the socket (the kind of fitting that lets you bend around things and reach into odd places that a straight line extension can't get into).
Oh, and it doesn't matter how long your "extension" is. As long as it stays 90 degrees to the handle.
Hope this muddies the water some.