Use the metal in a metal core PCB as the GND layer
Theoretically not. By default, via enable the connection between the layers, but technologically they must be protected against contact of the vias with the metal core, because then it would cause a short circuit on the board (not every vias you want to connect with a metal core).
But theoretically, you can order a PCB with mounting holes through all layers (copper, insulator and metal core) and then you could connect such layers with, for example, screws or maybe some solder.

Source
But it's still best to ask a specific PCB supplier, because what will be impossible for one, for technologically expensive ones, it is possible to implement.