I guess part of the argument against it is the simple logistics of it - I haven't seen the transaxle of a manual Aveo, but most of the manuals I've worked on don't have a "pan" in which to install said sensor? If you were to install it in the drain plug location (about the only spot likely to be threaded on the bottom of the trans), you probably wouldn't get a accurate reading, as the circulation of oil in the trans is simply done by splashing it around (in essence, I realize the gears will carry oil around).

Also, since there is no interface to a cooling system, about the only recourse you'd have in the event of a high temperature would be to pull over and wait for the trans and oil to cool down - which would likely take hours.

That all said, I don't think I'd be too concerned with hauling the sort of weight you're talking about up a hill - I'd be more concerned with going back DOWN the hill, and trying to safely control your speed. The brakes aren't really designed for that sort of work, and even a small trailer will end up with a "tail wagging the dog" sort of issue, at least possibly.

Just my 2 cents worth - I really don't have a dog in this race, as I have a larger vehicle for towing (diesel crew cab dually, as I don't think the Aveo will budge a 40 foot fifth wheel......)