Currently, there is no pump, nor tank, compatible with molten metal. Please either make the vanilla ones work, or make newer ones. I'm trying to use trains to ship molten metal (like we do IRL) but it is currently not possible to do so.

I tried adding molten iron and molten copper to the list of inert fluids, and got the following error:

The mod Call a Plumber (1.1.1) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event call-plumber::on_configuration_changed
call-plumber/scripts/registry.lua:53: Found an unexpected entry in the fluid category map registry for molten-iron
stack traceback:
[C]: in function 'error'
call-plumber/scripts/registry.lua:53: in function 'check_not_key'
call-plumber/scripts/registry.lua:105: in function 'register_fluid'
call-plumber/control.lua:77: in function 'register_custom_fluid_categories'
call-plumber/control.lua:86: in function 'register_defaults'
call-plumber/control.lua:109: in function <call-plumber/control.lua:106>

It was a deliberate design decision to limit how you can transport superheated fluids. The Inert Fluids setting wasn't designed for fluids that are already recognized by the scripting code. This error happens because this combination of mod settings directed Call a Plumber to classify molten-iron as an inert fluid and to classify molten-iron as a superheated fluid, and it can only do one of those things.