Hi

I might think that, while a worm drive might have some benefits, it might be a bit slow for good control unless it was a very fast ratio and thus not self holding. As I remember the foundry pourers had to cut the flow quickly and then reposition and start pouring again with little lost motion or time(temp) lost. That previous brake drum casting video has some ideas to learn(+ some to forget) as I am sure you have done.
A normal reduction drive like a gear or chain is simple enough and if the load is balanced not hard to control for accurate pouring. Then you can get/make an automatic friction clutch holding device for it, like you can get on a hand winch. It will drive forward and hold but drive back without having to lift a pawl or latch. I could shed some more light on these if you have not noticed them.
The spout will not be at the top will it. There will be some chamber above the metal for heat transfer and adding material and adjusting the mix, then deslagging and other necessary things ??
cheers Jaahn