Thanks all. I opted for the temporary splice as a repair to get it done before the rainy season started (i.e. today). Here's a few pictures...
The problem, with the engine running. I removed some of the rotten rubber from the bracket and its pretty much squirting ATF now.
Here's a good picture of the damage. You can see the hole in the cooler line -- its not quite as big around as a pin.
The completed repair. The engine's running and not even a drip.
I don't think this solution will last as many years as the original did, so I'm considering it to be temporary. All it needs to do is last until next summer, when I can replace the line altogether.
I don't have pictures, but it looks to me like the bracket was intentionally crimped, probably to tighten it down on the rubber and the tubing. Unfortunately that crimp appears to be the root cause of the problem. It created an inward protrusion that eventually rubbed through the rubber and then the metal of the line. The other side is the same, but not leaking yet. Like JHZR2 did and whunter pretty much recommended, if I get a break in the rain, I'll be replacing the rubber in that one too.. before it causes an identical failure.