I dont know what the ohm readings are, but the temp sending unit for the factory gauge on my 79 cherokee is a very small (like 1/4in dia) sender. Its a one wire sender though. Also the sending unit for the gauge on my 93 wrangler is the same size, but dont know the ohm reading on it either.

Neat idea about replacing the drain plug with a temp sender and using the trans temp input. Can you change the values to what temp would trigger a check engine light? Only problem I see with that, is you wont know what triggered the check engine light untill you scan it. I think I would prefer a gauge myself.