It should hold, trans temps can get up to what oil runs at or surpass it. Also there is most likely more pressure inside the unit as a trans cooler than there would be if it were used as an oil cooler. I am not positive on this, but it could be worth giving it a try or researching it a bit further, as far as trans pressure and such.
I can call my local radiator shop and see what they think, he has been around forever and repairs, rebuilds , customizes radiators for just about anything from diesel big rigs to cars.