I just came across this info about a low melting point tube-filling alloy for bending thin wall tubing, called Cerrobend:
http://www.hitechalloys.com/hitechalloys_005.htm
Definitely interesting. I have read about filling tubing and megaphones with molten lead before, but this seems a lot safer and easier.
Sounds like a kind of expensive material, but interesting. The link specifys using it on thin-wall, and probably best for the smaller diameters. Thin-wall is the street name for Electric Metallic Tubing, also called EMT, which is steel tubing specifically made as a conduit for electrical wires. Not suitable for exhausts.
We were all given this booklet during our 4-years of class when we were apprentices in the electrical union.
This booklet is all about using foot benders on thin-wall. The bender can make a code diameter 90 degree bend in a single shot. This page shows how to use a hickey (a slightly different kind of foot-bender) to make larger diameter bends.
In addition to thin-wall, we bent IMC (Intermediate Metal Conduit), and Rigid -- that was the heavy stuff, and anything bigger than about an inch required either mechanical or hydraulic benders.
We also bent electrical grade schedule 40 PVC. For small stuff, we used a heat gun. For the big sizes, say 2" and up, we had these electrically-heated oil baths. You'd put the conduit in the hot oil for a few moments, then pull it out and bend it.
If you were in the field, no electricity, you could put one end of a PVC pipe into the exhaust of a running work truck, and heat it up for bending that way.