My
handle_schematics mod does that in some form already. There are no particular ghost nodes for individual blocks though. There are just two nodes: The "dig here" indicator (a red pick) and the scaffolding node.
You can get a schematic beeing built out of these nodes if you place a handle_schematics:build node, revoke your creative priv and select one of the schematics. It looks like this:
When you right-click a "dig here" indicator, the node below will be digged and added to your inventory until enough room has been made for the schematic. The scaffolding node works likewise: Right-click it repeatedly. As long as you have the nodes required at that position in your inventory, they'll be drawn from your inventory and added to the building. The scaffolding node also shows you which node is needed at that position when you point your mouse at it. If you right-click a scaffolding node that has further scaffolding nodes below it, it will automaticly serve the scaffolding nodes lower down first, building from bottom up.
I'd love to have NPC running around and building these structures together with the player. Each party just has to right-click (or call a function) in order to help building. The NPC would need some additional behaviour: Go there, stare a bit, wield the right node (or digging tool), show digging animation, actually call the function, stop wielding the node.
The scaffolding structure just looks more natural to me than any ghost nodes. And with a ghost node you'd also run into the problem that there's a node already sitting there when your player comes and wants to place one.
