A great job your've done there.
From what I can gather that Metal Mend is resin based product with metal as filler. JB offer some similar stuff, as well as 2 part epoxy based (with talc and metal fillers).
Epoxy is a lot stronger when flexed than polyester based products. Polyester based body filler is even more brittle because of the filler used. When epoxiing, it is best to choose an appropriate fiberglass to reinforce. I use 540gsm unidirectional to laminate snowboard cores. If the core didnt snap first, it would almost be possible to bend it around in a 180 degree bend. I vacuum bag it to remove air and degass it. It also removes eccess resin so its not as thick and therefore bends and not split/crack.
Anyway, back on course. What about soldering. They used to do that to the c pillars to rear quater back in the days. In fact I saw on Gasolene once, the guys make up a whole corner (from light bucket to top, bottom and side) of a TR4 ??? front gaurd from solder which was then filed and sanded back. It looked smick.