Given this is still a very rare catch (by all accounts, somewhere between Mr. Pinchy and The 1 Ring), proving it cannot be caught in one body of water is very tricky indeed. If the drop rate is down below 0.1%, you'd have to cast many thousands of times in each location.
Generally Azeroth's fish catches follow a very simple pattern. In fact, with a few exceptions, there are only 6 variations in the whole of Azeroth. These 6 geographic variations are multiplied by an inland/coastal split, and in 2 of the 6, a further 4-option variation by time of day. This is the genius of the original design: They managed to create apparent complexity while retaining a very simple design. It's a very old-skool Elite-style approach to game design, but I guess works well here because it allows the game to be rebalanced very easily.
Now there are several location-specific variations - mostly obviously in The Barrens. However, Hillsbrad Foothills and Ashenvale never traditionally had any such variations. So, while it is possible they specifically divided an area like Hillsbrad up into local areas just to add the Darkwood Fishing Pole, I doubt this. More likely, the pole has simply been added to the "catch table" for all the inland open water in set of zones. I'm still not sure about the reports of catches from pools: It is impossible to verify the source (this is not shown in the combat log), which means that catches that appear to be from pools may not be (bobber actually just missed, lag delayed the disappearance of the pole, etc), which means lots of reports are needed. It would also completely inconsistent with the way pools generate loot: All the random items are in trunks, not floating about in the water.
But then, until reading this thread, I never knew about the Broken Wine Bottle (thanks Calabar). So keep on looking, and see what you find.
