My bathtub's shower diverter spout leaks when it's in shower mode, so I'd like to replace it. It's the screw-on type. Trouble is, it's stuck tight. I've applied a lot of torque (counter-clockwise) with a pair of channel-locks and it simply won't budge. I'm worried about snapping the copper pipe if I go any harder. The actual joint is deep inside the spout so there's no access to spray penetrating oil or something in there. Am I gonna have to grind it off with an angle grinder or something to access the threads?
Quite possibly. I have cranked on spouts like that and had them release, and had them fold up and crumble, since it was old pot metal. A grinder would be a good way to go about it. The connection, if it is as you say, will be in about 3" in. The rough in for a connection like that is usually 4" to 4 1/8" to the end of the male adapter, so a cut about 3" should get you to the adapter.
Yup, not uncommon. You could try cutting it like this: Then soak the threaded "boss" real good and carefully slice it with a hacksaw, preserving the threads underneath. I have had some success with this and it allowed speedy replacement without changing the wall nipple. Although if the nipple is not brass you should make efforts to get rid of it.
When building, I would never stub copper out with a male adapter for this type of spout, it's a recipe for a future twisted pipe.
Your right about over torqueing and snapping the pipe. Try this before cutting: 12 inch pipe wrench positioned on spigot for removal, 3/4 inch x 12-14 inch steel "cheater bar" onto wrench handle, pull wrench with medium force and release, position wrench opposite direction and repeat. Try doing this a few times.
If it comes down to it use a sawz-all before a grinder. If you make the cut at 45 degrees to the spigot you should be able to peal the spigot off with pliers.