Electroplating is exactly what you are doing to the electrode. Actually to be more correct its electro-accretion of minerals, but who's getting technical here?
. Once that outer layer forms it slows down. Also, it depends on how high you drive the voltage, whether you leave it unregulated, and so on. But really, the idea IS tho to plating limestone onto the growing medium, that's where the magic is.
The coral itself would not necessarily attach to the metal. It would attach to the accretion layer which can be (easily) chipped off. All the metals that I would spec would be marine safe, so you could grow into those directly. Also, you don't need a metal to be conductive. You could easily grow this onto a carbon as well.. just something to mull over there.
Give me a week to gather a bit more info and I'll let you know what I come up with.