You can get a kit for like $5 or so that lets you add a drain plug to a auto pan if it doesnt have one....drill a hole...after removing it and attach it with a nut on the inside.
If the sending unit is in the fluid that the pump is pushing out to the cooler and it is as close to the trans as you8 can get, about 6 inches in my case, being pushed by the pump, I seriously doubt it loses more than a degree or 3 by the time it hits the sending unit...
