I’m going to go with “sounds neat but is totally implausible.” Not totally implausible, but it’s a lot of work just to roll a bunch of unpredictable dice with the chaotic behavior of the boiling water and the chemical composition of the asteroid.

The water vapor is going to find the path of least resistance and come erupting out of the molten metal, probably quite explosively and you’re going to loose a bunch of valuable mass in the process.

EDIT: There may be a way to do this and get more predictable results (if you have a ton of energy to melt the asteroid very fast and very uniformly from all directions at once) but it still sounds unnecessarily complicated. There are simpler, cheaper ways to get better, more controlled outcomes.