Honestly, my guess would be that it would go off fine and you would get low velocity out of it. Black powder needs a full case or some slight compression to function properly, and I would guess that even FFFFg isn't going to cause anywhere near the pressure of a correct smokeless charge. That said, I'd be willing to bet someone else's gun on it, not my own or life and limb.
I got some .44 Magnums from a guy trying to give them to a local store for disposal. They were all corroded and the store didn't want them. He assured me they weren't handloads and that he had bought them, but no longer has a .44 mag. Inspecting them at home, they were several different headstamps with what appeared to be all the same bullets. Suspect. Maybe remanufactured ammo. Either way, I decided there's no way I'm shooting them and instead I'll pull them for components. It's just not worth it to mess around when you're containing a small explosion right next to your face and hands.