...
This sucks terribly, especially for someone who already commissioned a door sized drawing
...
What if we show them the artwork in advance?
Just an idea from a fellow attendee, here...
If you wish to show it off, you might consider getting a panel in the Art Show (that is, assuming that any panels are still available for purchase) and hang it there, on a NFS basis, for all to see!*
Myself, I have always missed seeing all the door signs and decorations because I never spend time cruising
all of the hotel room floors and halls (and for some conventions, it's multiple hotels!), but if you paid an artist good money for such a large door sign, you must have wanted people to see it, and this may be your only option. And if the artist who created it doesn't already have a panel, perhaps he or she might be willing to share the cost of one, just to show off their mad art skills...
...And you may still put a more appropriate-sized version on your door, to act as a location marker.
*NOTE: I may haunt the art shows of various conventions, but I haven't ever needed a panel myself; so I do not know which conventions charge for panels, and which offer them free to artists, in exchange for a percentage of the art sale price.