If you just want to make a quick/simple bend, and are not too concerned about anything being left inside your pipe, then just cut a length of sapling that will slip inside your pipe. That will provide the inside support as you bend it. After you bend it, you can try to fish out that sapling, but not really necessary from the sound of your intended use. Quick. Simple. Cheap. Just go slow bending the pipe.
Using DRY sand really works. We did a modern art blacksmithing project involving bending/twisting iron pipe. We welded caps on one end, filled with DRY sand, and capped the other end with a small hole drilled in it to allow any moisture out. We then heated 6 foot lengths of that pipe in a forge, and then proceeded to bend/twist/curl those pipes in and around each other to form the "tree" object we were working on. They formed the corners of a bed frame. We left the ends long, and then cut them off to length when done and cool. And the sand poured right out. We really "pushed" the limits with bending those pipes. Some sections looked like a ball of earth worms! Turned out well. And the client LOVED IT.
Yeah, that dry sand works well. But if you are heating your pipe full of sand up, be sure that sand is DRY! Any trapped moisture will turn to steam when you heat things up, and could lead to an explosion. Steam can build up to pretty high pressures.
But stuffing your pipe full of green saplings (one large sapling or a bunch of small shoots) will provide that internal support for bending without collapsing.
Mikey - that grumpy ol' Blacksmith out in the Hinterlands