FB bodies are useless pieces of ("Forgotten beast platinum") that cannot be interacted with in any way.
No, I'm quite positive that body parts of metal creatures can be melted. On the top of Iron man wiki page there's a mention of that. If it works for iron man, it should work for FB. Still, I never tried it myself, so I can't be completely sure.
The Iron Man monster becomes an Iron Statue item on death, which can be melted. It's a special case. All forgotten beasts become Forgotten Beast Corpses upon death, which are not valid meltable items.
As for solving the problem of a Forgotten Beast, Obsidian Traps works as well, though it requires that you have the ability to deploy both magma and water into a confined area containing the beast in fairly rapid succession. Killing beasts of any sort is easiest with bisection and decapitation, so use blade weapons, preferably swords, blunt weapons aren't very much help. If you can capture and tame giant spiders, then setting up a spider on a rope down a hall of cage traps and surrounded by traps leads to the FB becoming webbed on top of a cage trap, which is a condition that renders them vulnerable to capture in a cage, which is good fun in and of itself.
At this point though, you may want to cut your losses. Forgotten Beast poisoning is no joke, and letting those covered soldiers get back to the fort might be the end of them and the rest of your fort. It might be more merciful for all parties involved if you sealed them up and let them duke it out until the Beast dies or the Dwarves starve.