[me and my metro]

If the diagram in post 3 is correct and the resistance chart for my 4t40 is right what I would do is add a single pole dual throw switch. This is a switch with three posts on the back. The center post goes to the computer, one of the outside posts goes to the factory temp sensor inside the transmission. The other outside post goes through your resistor to ground. The switch has two positions so it is either one way or the other. So if overdrive works over 110 degrees then a resistor with a value of 400 ohms would make the computer think the oil was 175 degrees or so.
Your car is pretty smart though and it may not like seeing the trans oil temp at 175 when the cold soak tells it everything else is 60 degrees. I would drive it a mile or so before flipping the switch, this might avoid a check engine light.
The computer is also looking for overtemp conditions also, you will be taking that away with the resistor.