Looks like my idea was shot down and killed by forum regulars.
Foraging needs to be fixed in some way, though. It has been mentioned a long time ago, that you shouldn't be able to find sulphur in Hellice Lake or willow bark inside Hammerlorne.
Not killed. We're all discussing pros and cons. The only one who can kill an idea is Thomas, and he's not weighing in on these discussions that I've seen. He's lurking and trying to discern what sorts of changes he can reasonably make to please people without upsetting too many other people and without too much monkeying with the engine.
I do agree that Foraging finds should be delimited similarly to random encounters. Certain items should almost never occur in some places and be more common in others. Your one specific example I might disagree with--sulphur in Hellice Lake. Sulphur springs occur all over the place, and with all of the craggy mountains I would say there's lots of volcanic activity deep beneath that area. Some of the plants would be much less likely, such as belladonna. Those specifics would have to be figured out by Thomas for how he wants things to play out. He may give each possible item a weighted probability based on the area, so that at really high levels, you might be able to stumble on a stash of extremely uncommon material for an area, but never at lower levels.