Not to beat a dead horse, but this morning I did Science on the matter of stockpile erasure. Four times I built a wood pile, and while people were on their way to the wood, I erased said pile. And you know what? In that very instant the idler count went back up, and people went back to the meeting place. I even did it while one of them was carrying wood, and he dropped it right there and went back to other stuff. Every single time.
So, yeah, deleting a stockpile WILL cancel the job in an instant. At least with wood. Later on I'll do it with other stuff and see if it's dependent on material. This is in the latest build, 40.14. I remember it happening on earlier versions too. So, if this holds up, then you do have a way of cancelling hauling jobs. It's clumsy, but it seem to work. The one that doesn't work is disabling hauling in the middle of a job. In that case, they will finish the job before doing something else.
This in no way cancels the validity of the suggestion. Modders have done plenty of work trying to create this sort of thing. DF Hack has an alarm built in that cancels parties, but I don't think it cancels jobs in progress, or I would have mentioned it earlier.
Building on the earlier suggestion of dwarves passing on alarm information, I think that should be expanded so that any time a dwarf sees a change to the landscape/fortress, they should tell others that they pass. It'd tell haulers and the like when they need to change their path early, rather than letting them run into a wall or locked door and get confused for a split second.