There have been variations on this idea but I don't recall seeing exactly this setup.
What immediately comes to mind for me is the potential issue of condensed salt deposits within the arc tube causing some projection artifacts in an upside-down-vertical setup like this. I believe the salts tend to deposit on the gravity side of the arc tube and that would place them directly in your light path. Some members have encountered this problem with this kind of configuration.
Second is the volatile nature of attic space - temperature extremes are possible.
There may very well be other issues I can't think of prior to waking up this morning.
Still, cool idea and best of luck in your implementation if you go this route.