What comes to mind is some variant on "Pepper's Ghost" (Google it) It involves a large mirror that reflects a background that isn't there but appears to be solid. I have not given it enough thought to figure out the details to make it work, but there might be something there. As for the mirror part, look into "shrink mirror" (Google again) and other products like that.  It is a plastic material that you mount on a rigid frame, and then heat up with a hair drier or heat gun and it shrinks into a very tight decent mirror, if you did everything right. Rosco Shrink Mirror comes in a 54"x 30' Roll so you will get a couple tries out of it. 

I have in the past had great success with assumed walls. for example if the fireplace jutts into the room three feet the downstage side is seen but the upstage return is assumed. the mind just fills in the blanks like that. on the upstage wall we had bookcases and the end of bookcase closest to center was also assumed since the audience could not see that wall either, both of these were very effective for ghost to enter or exit. a well timed effect will add to the illusion.