Paracelsin mechanical plant takes molten iron input but cannot connect to Heat-Resistant pipes.
https://mods.factorio.com/mod/Paracelsin

Interestingly, it can connect directly to the foundry but not the pipes. This prevents crafting zinc pipes unless you hook the mechanical plant directly up to the foundry, which isn't an obvious or intuitive solution.

Even with the direct connection work-around, this would be annoying and problematic since you'd need to heat a foundry per recipe, but zinc pipes.

Zinc/Galvanized pipes apparently can carry molten iron without exploding, which allows the mechanical plant to connect to a molten iron fluid network.
This is actually a nice interaction and could work as an upgrade to let the player move past the limitations of hot-pipes with investment. (Paracelsin is post-Aquilo after all.)

Zinc pipes are intended to be difficult to export per the creator (rocket capacity of 10) and are very expensive, meaning hot pipes keep their value as the "cheap" option for molten fluid.
https://mods.factorio.com/mod/Paracelsin/discussion/68f91aafe3359475cfbc8e0c

Suggestion:

• Allow Mechanical plant to connect to hot pipes as the current behavior is unintuitive.

• Leave zinc pipe behavior alone, letting it act as an expensive alternative to hot pipe limitations.

Thank you for your time.

Additional discovery, the fluid passthrough on the mechanical plant does not appear to work with molten fluids. This is probably fine since it prevents breaking up fluid networks with the mechanical plant to extend the range.