I know that the 757 doesn't have fuel-dumping capability. Some countries don't like it over land, Canada being one - they were in the process of advising the Swissair MD11 when they could start dumping (ie after they'd got over the coastline) when the fire suddenly broke out behind the cockpit (until that point there'd been only smoke), at which point they started dumping anyway, even though they hadn't quite cleared the coast. I presume the dumped fuel disperses but I have a set of MEA 707 checklists that proscribe flying in circles while dumping, and there is a specific reference to the danger of flying through the trail of dumped fuel.
A friend of mine was the co-pilot on a 707 that suffered a major structural failure over the Alps, both engines on the right wing seperated, took out flaps, hydraulics and started a fire in the wing. They started dumping fuel, they'd actually taken off above max t/o weight so there was a lot of fuel to get rid of (flight was Luxembourg to Entebbe, Uganda), but the fire got worse and they had to shut the dump valves. Twenty minutes later they made a downwind landing at 220 kts at a French AFB called Istres and jumped out the cockpit windows as the wreckage exploded.
fly Saha Air 707s daily from Tehran's downtown Mehrabad to Mashhad, Kish Island and Ahwaz