as a guy who does soda blasting for a business i can tell you that you probably wont get the results your hoping for. it will remove the paint and leave it clean but its not abrasive enough to leave a uniform colored finish, so places that were stained with cooked on oil will still be dark. any pressurized blasting including dry ice will damage your seals, dry ice is 3x the price and you will get the same results as soda. i do alot of motorcycle engines with soda and tell the customer before hand how its going to look and most are happy.

that being said i wouldnt be afraid to blast a working tranny with soda. with a smaller tip you can avoid any seals that may be damaged