I have a spare T-5 I can sell ya so you can take it apart to weigh the gears and get their radii
Is the moment inertia of a cylinder 1/2*Pi*r^4? I'd just use the average radius of the gears and model the tranny as 1 big cylinder. Then make an educated guess (well, more like pulling it out of your ***) for the mass (~ .5-.75 slugs)
You would also have to also include the rotational drag that the tranny fluid creates. As well as the coeffiecient of friction for the clutch.....
arghhhhhhhh too much for me to think about