Remember that 1st gear is not synchronized. If the transmission output shaft is moving at all, you won't be able to get into 1st without a hammer and a lot of luck.
As atomarc mentioned, with oil in the transmission. it will act as a hydraulic coupling.
I suspect that it isn't transmitting much torque, but wouldn't try to stop it with my hand.
If you really want to know, wrap lots of cloth around the splines and grab the output shaft with a set of large vice grips. Attach the vice grips to a broom handle that will run into the frame if it turns with any force. Don't use a railroad tie.
What you want to have happen, if all else fails, is the broom handle break and make nasty noises until you put the transmission in neutral. A railroad tie is strong enough to cause expensive things to break.
