I had mentioned before using one of the pressure port fittings. No it won't be accurate as to what the in or out temperature is, but if idle (or normal) temp of the tranny is X degrees (regardless of where measured) then when you see it rise quickly, you know you have problems. It will measure cooling effectiveness IMO, but obviously won't give you in/out line temp.
If you ask me, since no one can give a straight, knowledgable answer on exactly what the trannies are SUPPOSED to run (I'd bet just over 195* or else it wouldn't be cooled in the radiator factory) temp wise, what it actually runs at is unimportant, as long as you KNOW when something is going wrong, or you need to give it a break, etc.
Putting the temp sensor in the pan is not really any better than the pressure fitting IMO, since the fluid is co-mingling at that point.
I don't like cutting tranny lines, that fluid is under a fair amount of pressure, and likes to leak.