I'm surprised that I haven't seen mention of Cerrobend here. It is a low melting point alloy designed for the purpose. IIRC, it melts below the boiling point of water, so you can extract it without getting hot enough to generate scale. The one disadvantage to Cerrobend (and several other Cerro alloys) is that they are quite expensive, so if you need a large amount for your support, it will not be cheap. (However, it is infinitely reusable -- just make sure that you melt it in a non-metalic container -- porcelain, quartz, anything which will handle boiling water and not be likely to form an alloy with your bending support.
As for the solder melting -- that may be well below the melting point of the alloy which you are using for bend support. *Good* solder is (or was) something like 38/62 (I forget which side was tin) but that was the percentage for the eutectic alloy -- the one with the lowest melting point.
Enjoy, DoN.