A man by the name of Andrey Rudenko has constructed a 3D printer for printing houses based on this idea. He's successfully printed a (small) castle and intends to print a house next. In terms of the direction you're going, he'd be the best person to talk to. You can read his blog and get his contact address at his web site, www.totalkustom.com.

In terms of scaling up, it seems the concrete mix and the deflection associated with scaling up the length of the railings are your biggest issues. I believe someone is addressing the latter by using small robots as mobile print heads, rather than the gantry approach.