I went underground with a personal roboport and a bunch of construction bots in power armor and tried to place a ghost entity near me. I got some complicated Lua error and subsequent game exit. It was apparent from the error that placing a ghost entity was supposed to fail.

Yeah, the same happened for me when I tried to place a ghost by shift-clicking...

With version 0.4.0, if the entity is allowed underground, you should be able to place a ghost of it as well.

Ghosts and robot construction of transfer entities (stairs, belt-elevators, and power transfer poles) are now also supported.

Cautionary note: if you place many ( >100 ) ghosts of of transfer entities then you will see a slight performance hit due to the logic which tracks those ghosts.