How about filling with melted paraffin wax and then just melt it out
after bending?

I don't think that would be strong enough, I think it will just extrude
and the pipe will flatten. Molten lead is another option; there used to
be low melting point alloys specially for this (Cerrobend, in the UK).
Non-trade name is Woods Metal.

works just fine if you start with tube a bit longer than you need,
fill 1t 100", and crimp both ends of the tube shut before bending.
Also works best if the tube is at temp where the wax is still
"Plastic" - not totally hard.

Full of grease works too, but it is a lot harder to clean out.
Tallow would likely work too, and it never gets as hard as Paraffin. -
again, perhaps not as easy to totally clean out??

Fair enough if you have spare tube to play with.